SSBC brake users read this !

[firefighter3931]

OK, i've had the SSBC force10 kit on my car for several years and have never been happy with the performance. I swapped master cylinders (Wilwood 1in bore) and it did improve a little but not quite satisfactory. I decided to grab the bull by the horns and really dig into the brake system to see what was causing the poor braking performance.  

The first thing i did was to purchase a brake pressure kit which would determine how much line pressure i was developing at the front caliper. I installed the guage and hit the pedal as hard as i could with one leg and the guage stopped at 800psi. Keep in mind the magic number to achieve lock-up is 1200 psi for disc brakes and 600psi for drum brakes.   So, the question then became...why was i not achieving the desired pressure. 

Research showed that with a 6:1 pedal ratio and a 1in bore master cylinder the resulting pressure would be 1200psi with only 150lbs of force on the pedal. I was putting in at least double that and still 33% short of the target pressure with twice the pedal input. Why was this happening ? 

I spoke with my buddy Dwayne and he came up with 2 possible scenarios...one of which i had considered, the other not. The first scenario was that the piston was bottoming out in the master cylinder and not moving enough fluid to adequately supply the calipers. That's what i was thinking, initially. The second scenario was something i hadn't considered ; the rear brakes were hydrolocking. I had never thought of that as a possibility but it sure could happen.

Let me explain ; consider that the same solid bar that actuates both circuits in the master cylinder is responsible for building pressure. Now, if for some reason one of the circuits couldn't move brake fluid.... that would limit plunger travel and affect the pressure in the other circuit. Seems plausable but i'd never encountered this before. 

Next phone call was to HP Mike who has installed several SSBC kits for customers and claimed that they work great...no problems. I quizzed Mike on his setup procedure and everything he does is the same as i had done. I asked about the adjustable proportioning valve and he said that he opened them all the way and road tested the car....choking back the adjustment until he is able to achieve simultaneous lockup on both front & back. At this point the little light bulb in my head turned on and i began to suspect the proportioning valve was an issue. This was based on the aformentioned hydro-lock theory explained above.

I then phoned the Willwood tech line and outlined the problem. My first comment was that with a 6:1 pedal ratio and their 1in bore master cylinder i was only able to generate 800psi of pressure at the caliper. His response : What's wrong with 800psi ? I stated that the car wouldn't stop for crap and the math shows that this combo of parts should be able to to make 1200psi easy ! His response : Just go with a 7/8 bore master. Really ? That's just a bandaid fix and not addressing the root cause i thought to myself. I then asked about the possibility of rear brake hydro-lock and my theory was readily dismissed. Impossible he said, 2 seperate, independant circuits and no way can it happen. That was the end of that conversation and i thanked him for his opinion and advice.

So, on a hunch i went back to the car and opened up the adjustable prop valve all the way and guess what : 1200psi !!! You've got to be friggin kidding me !!! I went back to the SSBC installation instructions and reviewed the prop valve setup procedure. The literature clearly states that the valve is to be opened 1/2 way and adjusted from there. I can tell you that this is totally WRONG ! Of course, mine had been adjusted 1/2 way open ! 

I informed Doug (R2) about this discovery because he had been complaining about the same issues using the same kit. He immediately went out to the garage and opened the prop valve to wide open and took the car out for a drive. Doug was so excited he called back within an hour and said the brakes were waaaaay better....night & day difference ! He hadn't been driving the car much due to such poor brakes and didn't feel confident the car would stop in a panic situation. His prop valve was adjusted as per the SSBC procedure and was still 1/2 open or 1/2 closed...same difference. Didn't matter because it was wrong. 

So, based on the research, some old fashioned analysis & pressure testing along with a little common sense the problem is solved. I'll bet that the majority of guys who've installed the SSBC kit and followed the manufacturers installation instructions have  been having the same issues as I have. There's nothing wrong with the kit but the prop valve set-up instructions are flat out wrong. If you have a SSBC kit and are experiencing poor brakes my recommendation is to open the prop valve completely and road test. If the back brakes lock up before the fronts you can adjust the bias slowly by decreasing pressure at the prop  valve....in very small increments.

Worth noting ; to increase the line pressure you need to turn the knob clockwise. To decrease it, rotate counterclockwise. This does seem backwards when you're looking at the adjustable prop valve but trust me on this.  



Ron

[Fred]

I have SSBC and I agree with you fully. I made the exact same adjustments to my system and it's never failed me. As I said before, I run it with the hydraboost
I couldn't be happier.

[tan top]

good bit of detective work Ron    , thanks for sharing your findings 

[Dino]

Once more Ron to the rescue.   

Sticky please!

[Ranger Max]

.

[Just 6T9 CHGR]

When I bought the standard 4 piston cast iron manual front disc brake set up from SSBC, I complained to them from the get go that the pedal effort to stop the car was waaaay to excessive over the previous manual drum set up.  I was told it was "inherent to the design" of the disc brake system due to the fact that the caliper has to "squeeze" the pads and witha drum setup it is a cantilever effect.

After living with that answer for a year I finally wound up adding the stock Bendix power booster setup to the car & the brakes work ok since then but could there be room for improvement with a prop valve adjustment?  

I might just try it out!

[BBKNARF]

Thanks for the info Ron, I'm running SSBC front manual disc brake system and when I brake I really have to push the brake paddle down hard, I will try it. 

Frank

[1970Moparmann]

Way to go Ron.   Your a wealth of knowledge on this site!  Keep up the great work...

[R2]

Ron, you da man.
Great post, and my brakes are now working great. 
I spoke to Dwayne last week, and he reviewed the research you guys did,,,nice work....

[66Auto-xcharger]

Ron, I will Try you adjustment Next summer as I have all ready put the Charger up for the winter. I have The SSBC Force 10 front and rear disk set up with Power running 21" of vacuum, down right scary to drive in town.  What's fun is Auto-X At this time I have never gotten into wheel lock up. Most cars that I have driven with stock brakes would be into 4 wheel lock up, with half the pressure that I am applying to the pedal just to slow the Charger down. By far this has been the worst brake set up on the Charger.

Worth Noting This may be my problem The adjustable prop valve I may have been turning the knob Backwards this would explain the lack of braking.   

David H.

[66Auto-xcharger]

OK I went out and checked the Adjustable Proportion Valve and found like I thought that it was turned all the way Counter Clockwise. So I turned it all the Clockwise Hoping for dry with to day so I can take her out and test the brakes. 

[Rolling_Thunder]

I never had a problem with my SSBC discs...          Then again that's because I never rear the instructions - I just bolted it on and opened the valve all the way - stopped good so I never screwed with it further 


Moral of the story?   Don't read the instructions     

Good to know there is a root cause to the problem though - thanks for the research   

[charger Downunder]

The literature clearly states that the valve is to be opened 1/2 way and adjusted from there.
Ron

It looks like you finally got around to the adjusted from there part.

[66Auto-xcharger]

OK I adjusted the Proportion valve the other day. It Stopped raining finely so I took the charger out to try the brake No Difference. I Can not lock the Brakes on a wet road, it just slows down as if I was only applying light brake pedal force. So next spring I will get a Gauge and see what pressure is being generated at the Calipers. Here is quick run down what has been done so far. All ending result with the same problem.   
1.   rebuilt all 4 calipers
2.   replace Adj. Proportion Valve
3.   Eliminated factory Proportion Valve and metering Block
4.   Replace Master Cylinder
5.   All new SS brake lines
6.   New Braded Brake hoses
7.   4 brake boosters two were new
8.   Two sets of Brake pads  one Stock non metallic and one EBC Green Stuff The stock worked better on the Street 

David H.

[firefighter3931]

OK I adjusted the Proportion valve the other day. It Stopped raining finely so I took the charger out to try the brake No Difference. I Can not lock the Brakes on a wet road, it just slows down as if I was only applying light brake pedal force. So next spring I will get a Gauge and see what pressure is being generated at the Calipers. Here is quick run down what has been done so far. All ending result with the same problem.   
1.   rebuilt all 4 calipers
2.   replace Adj. Proportion Valve
3.   Eliminated factory Proportion Valve and metering Block
4.   Replace Master Cylinder
5.   All new SS brake lines
6.   New Braded Brake hoses
7.   4 brake boosters two were new
8.   Two sets of Brake pads  one Stock non metallic and one EBC Green Stuff The stock worked better on the Street 

David H.

Dave, what is the bore diameter on your master cylinder ? What brand is it ? Did you install a residual pressure valve(s) ?

In theory your brakes should be fantastic with that combination of parts. 


Ron

[mauve66]

 

[RECHRGD]

Great find Ron!  I did the standard SSBC manual front disc conversion several years ago and have never been happy.  I changed the 1" master to a Wilwood 7/8" and it helped, but will still not lock the brakes up.  The car is now up on the lift in the shop for the winter, so no testing until Spring.  I do remember setting the valve at about half open as they suggested.  YOU ARE THE MAN!

[Dino]

I'm getting doubts on installing my disc brake kit now.  I fiddled with the old drums a bit and now it stops on a dime.  Go figure...

[BLUE68RT4ME]

Good info Ron!  I think this is the first thread I've read that you have started.  After reviewing your post, your thought process and way of explaining things has a scary resemblance to the way I think.  Yikes!  lol

This is good info for me to take in.  I would like to get my beast to stop a little quicker than it does now but don't want to make the move to disc brakes... it's the "appearing stock" thing for me.  Can you describe your entire set-up?

Thanks again!

[firefighter3931]

Good info Ron!  I think this is the first thread I've read that you have started.  After reviewing your post, your thought process and way of explaining things has a scary resemblance to the way I think.  Yikes!  lol

Hi Mark, your 68 is lookin great !!! I don't start many threads....mostly troubleshoot & answer questions and PM's    This thread was wortwhile because so many members have complained about the SSBC kit over the years....

 
Can you describe your entire set-up?

Thanks again!

Mine was originally a power drum car similar to yours (don't know if you have a PB booster) and i decided to go manual brakes due to low engine vacuum and not being able to run a booster with the big cam. The conversion kit bolts right onto the factory drum brake spindles which is a huge bonus. At the time SSBC was the only company making that type of a kit. Front rotors are slotted 11in diameter and the calipers are fixed (non sliding) 4 piston aluminum units. This kit fits inside a 15in wheel and that was a requirement for me using the Weld Pro-Stars.

Since this kit came out there have been several companies come on board making similar kits and some with larger diameter rotors (increased stopping power) for those running larger wheels, like yourself  With the larger wheels you have there are several options for big brakes and with the design of your Magnum wheels you wouldn't even see the rotors/calipers. I understand the stock appearing concept but Brakes are something i don't compromise on if there is improvement to be had out there. 

That being said ; an 11x3in drum brake can be quite effective on the initial brake attempt but drum brakes will fade when heated up so if you have to make 2 panic stops back to back there will be some loss of stopping power/brake performance. 

Seeing that mine sees some track duty i felt that an upgrade was in order....i now feel comfortable going through the traps at 125 and being able to slow the car down in a short distance. 

The braking system consists of a WillWood 1in bore master cylinder, line lock to the front SSBC disc setup. The rear brakes are 11x3 Factory drum brakes with a proportioning valve & residual pressure valve.


Ron

[RECHRGD]

Well, I really thought this was my problem also, but it doesn't seem to make any difference, if any in my case.  Maybe the change to a 7/8" bore has something to do with it.  Admittedly, I have not yet tried a panic stop, but during a few fairly hard stops it didn't feel any different to me.

[firefighter3931]

Bob, are you using the factory distribution block ? If so, was this a drum brake car originally ?

FWIW, I removed the factory block and have the front lines tee'd off from a line lock and the back brake line off the master goes directly to the prop valve. 

Ron

[RECHRGD]

Hi Ron.  Yes, I retained the original block and it was originally a drum brake car.  So I guess your saying you just did away with the block?

[Budnicks]

Good info Ron!  I think this is the first thread I've read that you have started.  After reviewing your post, your thought process and way of explaining things has a scary resemblance to the way I think.  Yikes!  lol

Hi Mark, your 68 is lookin great !!! I don't start many threads....mostly troubleshoot & answer questions and PM's    This thread was wortwhile because so many members have complained about the SSBC kit over the years....

 
Can you describe your entire set-up?

Thanks again!

Mine was originally a power drum car similar to yours (don't know if you have a PB booster) and i decided to go manual brakes due to low engine vacuum and not being able to run a booster with the big cam. The conversion kit bolts right onto the factory drum brake spindles which is a huge bonus. At the time SSBC was the only company making that type of a kit. Front rotors are slotted 11in diameter and the calipers are fixed (non sliding) 4 piston aluminum units. This kit fits inside a 15in wheel and that was a requirement for me using the Weld Pro-Stars.

Since this kit came out there have been several companies come on board making similar kits and some with larger diameter rotors (increased stopping power) for those running larger wheels, like yourself  With the larger wheels you have there are several options for big brakes and with the design of your Magnum wheels you wouldn't even see the rotors/calipers. I understand the stock appearing concept but Brakes are something i don't compromise on if there is improvement to be had out there. 

That being said ; an 11x3in drum brake can be quite effective on the initial brake attempt but drum brakes will fade when heated up so if you have to make 2 panic stops back to back there will be some loss of stopping power/brake performance. 

Seeing that mine sees some track duty i felt that an upgrade was in order....i now feel comfortable going through the traps at 125 and being able to slow the car down in a short distance. 

The braking system consists of a WillWood 1in bore master cylinder, line lock to the front SSBC disc setup. The rear brakes are 11x3 Factory drum brakes with a proportioning valve & residual pressure valve.


Ron

Just curious.... what pressure residual valve did you use for the rear drums & where did you install it in line ??

[firefighter3931]

Hi Ron.  Yes, I retained the original block and it was originally a drum brake car.  So I guess your saying you just did away with the block?

Yes, the distribution block is Gonzo ! 

Just curious.... what pressure residual valve did you use for the rear drums & where did you install it in line ??

I used a 10psi RPV and it is installed right behind the adjustable prop valve.


Ron

[Budnicks]

Hi Ron.  Yes, I retained the original block and it was originally a drum brake car.  So I guess your saying you just did away with the block?

Yes, the distribution block is Gonzo !  

Just curious.... what pressure residual valve did you use for the rear drums & where did you install it in line ??

I used a 10psi RPV and it is installed right behind the adjustable prop valve.


Ron

Thanks firefighter, I'm getting ready to do a very similar, drum to disc SSBC kit on my 68 RR, I had purchased a few years ago now... thanks for the heads up... I was think about just changing out the whole distribution/proportioning valve to a Right Stuff front disc & drum rear brake style distribution valve $82 & get rid of the drum brake style distribution valve... what do you think ?? would that help with the braking any ?? or is it a waste of money ??

[firefighter3931]

Thanks firefighter, I'm getting ready to do a very similar, drum to disc SSBC kit on my 68 RR, I had purchased a few years ago now... thanks for the heads up... I was think about just changing out the whole distribution/proportioning valve to a Right Stuff front disc & drum rear brake style distribution valve $82 & get rid of the drum brake style distribution valve... what do you think ?? would that help with the braking any ?? or is it a waste of money ??

I can't say for sure if that factory valve would work well or not but keep in mind it is designed for a car using stock parts which include a single piston caliper which has more clamping power than the 4 piston SSBC unit. The surface area of the single piston is bigger, believe it or not.

That being said, the factory bias might not be ideal with a 4 piston caliper up front ? 

I can tell you that mine works great without the factory drum/disc valve, or any factory distribution block for that matter. If it were mine, I wouldn't bother 


Ron

[Budnicks]

Thanks firefighter, I'm getting ready to do a very similar, drum to disc SSBC kit on my 68 RR, I had purchased a few years ago now... thanks for the heads up... I was think about just changing out the whole distribution/proportioning valve to a Right Stuff front disc & drum rear brake style distribution valve $82 & get rid of the drum brake style distribution valve... what do you think ?? would that help with the braking any ?? or is it a waste of money ??

I can't say for sure if that factory valve would work well or not but keep in mind it is designed for a car using stock parts which include a single piston caliper which has more clamping power than the 4 piston SSBC unit. The surface area of the single piston is bigger, believe it or not.

That being said, the factory bias might not be ideal with a 4 piston caliper up front ? 

I can tell you that mine works great without the factory drum/disc valve, or any factory distribution block for that matter. If it were mine, I wouldn't bother 


Ron

You answered my next question already, about size... Kind of what I was wondering... Thanks Ron/firefighter3931 again for the input, your very helpful, I will just ditch the original dist. block too & use a T for the front & the prop valve that came with the kit with a 10psi residual valve after it for the rear...

[Big Sugar]

I just installed the Viper 12" fronts discs setup and Dr Diff's rear 10.7" disc setup,  i used a mopar performance 15/16" aluminum master , i had already purchased a adjustable proportioning valve for the rear line but Cass (Dr diff) stated that i would still require a factory style distribution block and as i was running discs all round that i needed the 67 and up distribution block for a drum brake car, when i questioned him as to why i could not just split he front circut and run an adjustable out back , he stated to get a properly function system i would need the drum brake distribution block only but could add the adjustable to the rear circut should i feel i needed it.
  Should have some results later this week.


Ron

[firefighter3931]

Ron, a disc/disc install should not require an adjustable prop valve. Both the front & back calipers require the same line pressure (1200psi or more) so installing a valve that reduces line pressure out back will affect rear braking...and consequently the braking performance of the entire system. 

On a drum/drum car the factory block has no internal valving....it is just an elaborate "T" to tie the whole system together in one neat package with a warning switch that tells you when the brakes are malfunctioning. 

Let us know how you plumbed it and the results 


Ron

[Big Sugar]

Well ...Works Great Took as few miles for the pads to break in but after that it was Good !   Pedal came up no Mush, Great firm feel. With the proportioning valve I can turn down the rear pressure But so far its on full to the rear and the Charger stops very well. No concerns.   Guess I could remove the P V now but I'll see ho the Charger does in the wet later.  For now All is Good.

Ron

[dangina]

what brand of adjustable prop valve did you use?

[dodge5966]

 ;)Just read your post went out to shed adjusted valve and  hey presto brakes
Regards Mike

[LaOtto70Charger]

Firefighter at what one point in the pedal travel do you get 1200 psi on the fronts?  I have power brakes with mine and I only get 1200 psi completely at the floor and pressing really hard.  Brakes only really work in last quarter of the travel.  Can feel some pressure being applied before that.  Maybe.  But not enough to do anything.  I only get about 600 psi when brake fully applied without standing on it.  I plan onchecking master cylinder push rod this weekend but checking if I am missing something else.

Brakes have been bled.  New power booster.  Replaced rear rubber brake hose along with new wheel cylinders and shoes on the rear drums.

[ray230]

OK, i've had the SSBC force10 kit on my car for several years and have never been happy with the performance. I swapped master cylinders (Wilwood 1in bore) and it did improve a little but not quite satisfactory. I decided to grab the bull by the horns and really dig into the brake system to see what was causing the poor braking performance.  

The first thing i did was to purchase a brake pressure kit which would determine how much line pressure i was developing at the front caliper. I installed the guage and hit the pedal as hard as i could with one leg and the guage stopped at 800psi. Keep in mind the magic number to achieve lock-up is 1200 psi for disc brakes and 600psi for drum brakes.   So, the question then became...why was i not achieving the desired pressure. 

Research showed that with a 6:1 pedal ratio and a 1in bore master cylinder the resulting pressure would be 1200psi with only 150lbs of force on the pedal. I was putting in at least double that and still 33% short of the target pressure with twice the pedal input. Why was this happening ? 

I spoke with my buddy Dwayne and he came up with 2 possible scenarios...one of which i had considered, the other not. The first scenario was that the piston was bottoming out in the master cylinder and not moving enough fluid to adequately supply the calipers. That's what i was thinking, initially. The second scenario was something i hadn't considered ; the rear brakes were hydrolocking. I had never thought of that as a possibility but it sure could happen.

Let me explain ; consider that the same solid bar that actuates both circuits in the master cylinder is responsible for building pressure. Now, if for some reason one of the circuits couldn't move brake fluid.... that would limit plunger travel and affect the pressure in the other circuit. Seems plausable but i'd never encountered this before. 

Next phone call was to HP Mike who has installed several SSBC kits for customers and claimed that they work great...no problems. I quizzed Mike on his setup procedure and everything he does is the same as i had done. I asked about the adjustable proportioning valve and he said that he opened them all the way and road tested the car....choking back the adjustment until he is able to achieve simultaneous lockup on both front & back. At this point the little light bulb in my head turned on and i began to suspect the proportioning valve was an issue. This was based on the aformentioned hydro-lock theory explained above.

I then phoned the Willwood tech line and outlined the problem. My first comment was that with a 6:1 pedal ratio and their 1in bore master cylinder i was only able to generate 800psi of pressure at the caliper. His response : What's wrong with 800psi ? I stated that the car wouldn't stop for crap and the math shows that this combo of parts should be able to to make 1200psi easy ! His response : Just go with a 7/8 bore master. Really ? That's just a bandaid fix and not addressing the root cause i thought to myself. I then asked about the possibility of rear brake hydro-lock and my theory was readily dismissed. Impossible he said, 2 seperate, independant circuits and no way can it happen. That was the end of that conversation and i thanked him for his opinion and advice.

So, on a hunch i went back to the car and opened up the adjustable prop valve all the way and guess what : 1200psi !!! You've got to be friggin kidding me !!! I went back to the SSBC installation instructions and reviewed the prop valve setup procedure. The literature clearly states that the valve is to be opened 1/2 way and adjusted from there. I can tell you that this is totally WRONG ! Of course, mine had been adjusted 1/2 way open ! 

I informed Doug (R2) about this discovery because he had been complaining about the same issues using the same kit. He immediately went out to the garage and opened the prop valve to wide open and took the car out for a drive. Doug was so excited he called back within an hour and said the brakes were waaaaay better....night & day difference ! He hadn't been driving the car much due to such poor brakes and didn't feel confident the car would stop in a panic situation. His prop valve was adjusted as per the SSBC procedure and was still 1/2 open or 1/2 closed...same difference. Didn't matter because it was wrong. 

So, based on the research, some old fashioned analysis & pressure testing along with a little common sense the problem is solved. I'll bet that the majority of guys who've installed the SSBC kit and followed the manufacturers installation instructions have  been having the same issues as I have. There's nothing wrong with the kit but the prop valve set-up instructions are flat out wrong. If you have a SSBC kit and are experiencing poor brakes my recommendation is to open the prop valve completely and road test. If the back brakes lock up before the fronts you can adjust the bias slowly by decreasing pressure at the prop  valve....in very small increments.

Worth noting ; to increase the line pressure you need to turn the knob clockwise. To decrease it, rotate counterclockwise. This does seem backwards when you're looking at the adjustable prop valve but trust me on this.  



Ron

Ron,
I don't know why but I just found this posting this week. A few years ago I did the SSBC (A156) front disc conversion on my power drum 68. I have never been happy with the performance, almost to the point of being unsafe. I set the proportioning valve at the mid point per the instructions too.

After reading this post I opened it up all the way. Weather is crappy so only a short round the block, but I could instantly tell the pedal was MUCH better. Thank you. Thank you. Thank you for posting this!

[rsw0331usmc]

Have there been changes to the proportioning valve? I turned the valve clockwise all the way thinking I was opening it, then tried bleeding the rear. Couldn't get a drop out. Went opposite all the way and fluid was able to flow. The roads here are still wet from last week's snow so I probably won't be able to road test for a few days.

[RECHRGD]

Just for giggles I purchased the SSBC pressure gauge kit through Amazon.  Today I had some time and thought I'd go see how much pressure I was getting at the front calipers.  Got the passenger front wheel off and went to get the pressure gauge.  When opening the package the gauge went flying and landed on the concrete shop floor.  When I picked it up it was stuck on 700 pounds.    after the dirty words, I decided to go ahead and use the gauge just assuming that 700 would be my zero point.  With the proportioning valve set at full counter clockwise it showed a max of 2,200 then back to 700 every time.  I then turn the valve to full clockwise and the reading did not change at all.  I then went to the midpoint, per the instructions, and again, no change.  So if the gauge is reading right, I'm getting 1,500 pounds at the front calipers.  But, why no change with different valve positions?  I've never liked the hard peddle, but have made some pretty strong stops over the years.  However, the I've never locked the wheels and thought I had the low pressure issue.....

[ChargerSwede]

Old thread, old problem, new user.
Installed an A156-kit this year and experienced the same thing as the threadstarter, hard pedal and can´t lock the brakes. Adjusted the proportioningvalve as ts wrote and the feeling in the pedal was way better. Only problem is that the rear Wheels lock up before the front ones. Adjusted the valve to get the front ones to lock up first and then the stiff pedal came back.
What´s inside the distributionblock? Is it just a strange designed T or is there any valves inside it that can play us some tricks?

The car was originally equipped with powerassisted drumbrakes.

[LaOtto70Charger]

What master cylinder and power booster is on the vehicle? My car did fine at first with drum booster but it went bad quick.  Switched to manual and much better.

[Kern Dog]

My situation is similar.
1970 Charger, originally a 4 wheel non power drum system. In 2000, I swapped in an 11" front disc setup consisting of entirely USED parts I pulled from a 1975 Dart. Spindles/knuckles, rotors and calipers...even the hoses. The power booster even fit since it has the common 4 bolt mounting pattern. I used the Dart disc/drum prop valve. I still had the 10" rear drums. The car stopped great! About a year later I upgraded to 12" rotors from a Cordoba.
In 2006, I swapped in the Dr Diff 10.7" rear disc brake kit. The car stopped about the same but at least it looked better through the large spokes of my wheels.
In 2012 I wanted to clean up the engine bay and a switch to NON power assisted brakes was the goal. I had 3 manual master cylinders here to work with. First up I tried a MC I got from a friend....A unit included in a Master Power brake kit. The MC looked like a GM unit with large reservoirs and a 1 3/16" bore. The pedal feel was great, real firm but the brake force was terrible. I was not even confident enough to take it on the country road to test it. I then tried a slightly smaller 1 1/8" unit and it felt the same. Lastly, I tried a 15/16" unit from a Valiant or Dart. Again, firm pedal, poor brake force. I had no pressure gauge to confirm anything but it felt as if the amount of force I was applying did not translate to brake force at the pedal.
I read up on pedal leverage and although I currently don't recall the specifics, I was safely within the margins.
Then I called Dr Diff. He suggested a new 15/16" MC. I bought his, installed it and found the same result yet again. In what was a somewhat humiliating experience, I has to admit defeat and switch back to the power assisted setup.
A little time passed and I was under the car to do something.....I saw a Disc/Drum proportioning valve. The car has 4 wheel discs!
I looked through my parts stash and found a few drum/drum blocks and picked the cleanest one. The tide was turning. The car stopped better than it ever had.
I decided to test a theory and took a spare brake pedal...and drilled a pushrod hole above the stock one....MORE leverage now! Braking improved even further. The booster has a threaded adjustable rod inside that affects when the pedal begins building pressure. I adjusted it back and forth to get the brakes to react right near the top of the travel. I did go too far once and felt noticeable brake drag on a test drive.
Now I am back thinking about a manual MC setup again. Low vacuum is a problem for power brakes and I want the freedom to run a  wilder cam if the mood strikes. I don't want to mess with a vacuum pump or tank. I think that the majority of my troubles were due to using the disc/drum prop valve instead of the drum/drum block.

[thor]

I was thinking 20 in. Buy 10 inch wide in the rear and 18 in by 8 in the front? What are your thoughts on that? Rim size

[FI-Charger73SE]

Let's see what mine's going to be and what issues and experience.

Wilwood 1,x master cylinder with hydraulic booster, Wilwood 4 cylinder caliber front and Bauer 4 cylinder calibers in the back :-)

Petri