What can I do with my power disc brakes if I have poor vacuum?-UPDATE-FIXED!!!

[Mfr426]

Hey folks it's been a while since I posted. I had another daughter 3 weeks ago, finished a 6 month home addition and had a 9 month business project FINALLY come to an end so I've been doing everything but playing with my Charger. I currently have a dilemma that I need some assistance with.

I have a 69 Charger SE with a 71 383 "G" motor. I made the mistake years ago of buying the Mopar Performance "Road Runner cam" with the following specs: Hyd. - 284 / 284 Duration - 484 / 484 Lift. Keep in mind that this is a low compression motor (but is in very good shape). I have power disc brakes and they require a ton of pressure to stop the car. My buddy with a 68 RT with power DRUM brakes drove my car and said that the car was downright dangerous. He said that there is clearly something wrong. He seems to think it's the lack of vacuum from the aggressive cam. I'm inclined to agree.

Here's an interesting twist, I just got my stroker motor back after 5 months at the machine shop and soon I'll be installing this in my car. However, I will still be driving my car this summer until I can get the stroker installed. So, I need the brakes to work reasonably well until the stroker is installed.

My question is:  Do you folks agree that that cam in that motor would cause issues with power disc brakes? Keep in mind that all componets of the system are new (lines, booster, MC, calipers, pads, etc).  Second, does anyone have any ideas for a stopgap fix for this without replacing the cam in this soon to be replaced motor? I'll most likely take a vacuum reading this weekend but wanted to bounce this situation off the forum and the "experts".

I'll post stroker photos and details later this month.

Thanks all for any comments!

Mike R in Reading PA
69 Charger R/T Clone Project

[Charger-Bodie]

that cam really shoulnt create that much of a vaccum problem unless the gine is pretty warn out

[pandamarie]

I run that same cam in my 440 6 pack challenger and still have 21 inches of vacuum. with everything new I sounds like either a vacuum leak somewhere, check the whole headlight vacuum system, or if you didnt buy the MC and booster together the rod between them might be out of adjustment causing the loss of pressure due to a short stroke. open your MC cover and have someone push the pedal down and watch the piston through the holes, the seals should go past them, if its not its an easy fix, also did you bench bleed the MC to make sure there was no air in the system?

[Mfr426]

Thanks guys for the quick help. The booster and MC was not purchased as one and there have been several threads about that rod adjustment on this forum. The new MC's have plugs and new directions that indicate that you no longer have to bench bleed the MCs like you use to do. I'm hoping that it was the MC companies attempt to short cut the proceedure? I'll check for the range of that piston this weekend.

Thanks for the ideas!

[BrianShaughnessy]

What's the pedal like?    Long stroke? Does it go to the floor almost?   Does it pump up?     

[Just 6T9 CHGR]

I dont know how pandamarie got 21" of vacuum with the .484 purple shaft cam........myself including a lot of others were lucky to get 10" the most at idle

Mike did you actually check the vacuum at the booster?  "They" say a min of 19" will be sufficient to run the booster.

With my .501" Engle cam I am getting around 14-16" of vacuum and my brakes work beautifully.

I also agree with panda about the booster rod adjustment.  The way I did mine was to adjust the rod to get the slightest amount of movement in the master piston when bolting it to the booster.

In your situation I have heard of some 68-69 Charger owners tapping into the headlight vacuum can acting like a reserve...

if your manifold vacuum is not sufficent enough & the vac can trick doesnt work,  your next recourse would be to buy a vacuum pump.  MP brakes sells them but they are very pricey though


PS---ALWAYS bench bleed the master...trust no one! 

[Ghoste]

I have that cam in my 383 as well and I agree with Chris, 10 inches of vacuum is about the most.  That being said, it seems to stop my factory disc setup okay.  Nail the brakes a few times in a row and it becomes a little dicey, but...
No vacuum leaks anywhere, right?

[8WHEELER]

I have that cam in my 383 as well and I agree with Chris, 10 inches of vacuum is about the most.  That being said, it seems to stop my factory disc setup okay.  Nail the brakes a few times in a row and it becomes a little dicey, but...
No vacuum leaks anywhere, right?

  You are very lucky to get 12-13-lbs of vacuum out of that cam. I have run that cam most
of the years I have had my red car. I can't get 21-lbs out of some stock cams. I have tried the
vacuum can, did not notice any improvement at all, so MPB got my money for a vacuum pump.
Brakes will give you as many pumps as you need for great brakes, even if the engine dies,
of coarse, I would not know about that 

Dan

[Mfr426]

Hey Guys, when I checked vacuum on this about a year ago I had around 8 inches. I hope to recheck the travel of the piston in the MC this weekend. Pedal travel is about average and not to the floor. I bought an "EZ BLeeder" a few months ago and have yet to use it although I did the old "pump, pump, pump...HOLD" method several times. I may bench bleed it (again) but I dont have much hope that that is the issue as I did that (with the newer method) when I got the MC. I'd had this issue with the older MC too and I did the traditional bench bleed with that one as well.

Thanks for all the comments.

[Ghoste]

That sounds about right.  BTW, those specs aren't for the Road Runner cam, that's from the Street Hemi.  The Road Runner cam had more lift and less duration I believe.  Irrelevant to your question but in case you want to know.

[firefighter3931]

You will need to add a reservoir to help with vacuum as mentioned above. The 284/484 is larger than the old "street hemi" grind and has more overlap and a tighter lsa....both of which reduce manifold vacuum. The roadrunner cam will produce the best vacuum out of those three ;

Roadrunner cam specs :

268*/284* advertised duration
115* lsa
46* overlap
.450/.458 lift


Street Hemi cam specs :

280*/280* advertised
110* lsa
60* overlap
.474/.474 lift


484 purpleshaft cam specs :

284*/284* advertised
108* lsa
68* overlap
.484/.484 lift



As you can see....when the lsa is tightened up and the duration is increased the overlap also increases. The more overlap you have the less manifold vacuum it will make.  Other factors such as engine displacement, static compression and intake manifold selection will also influence manifold vacuum readings but the cam is a big "player" in the overall result. 



Ron

[bull]

Get rid of the booster and run manual brakes. Problem solved.

[daytonalo]

PIECE OF CAKE , THEY OFFER A  SMALL ELECTRO -VAC PUMP FOR THIS PROBLEM ! NOW FOR A REAL DELEMA , WORLD PEACE !!!!!!!!!

[Rolling_Thunder]

the .484 MP cam doesn't make vacumme at all...     i had one...    summarily removed it...        installed a bigger camshaft and went with a hydroboost unit...       maybe a vacumme pump is in your future ?

[Mfr426]

Hey all, thanks for the post and for the clarification on the cam name.

Now for the good news; I took a vacuum reading at the booster at idle with a warm motor and got a solid 15" of vacuum. I see that as good news!!! I know the motor has good compression (new rings, etc.) and this number tells me that my motor is in good shape and that vacuum is not my issue. My next step will be to make sure the piston is making the fulll travel (I'll need the wife for this test). The booster itself is new.

The only other adjustment is the rod that pushes in the piston. I remember we went over this months ago on this forum and many said that this was most likely NOT the isssue. At this point I'm going to check everything.

Thanks all for the comments...

[Just 6T9 CHGR]

Good news Mike.....read my post above on how I adjusted mine....

[Mfr426]

Hey Chris, I saw that comment about getting the slightest amount of movement when you bolt on the MC. The only problem that I had was trying to actually get that acorn style nut to turn on the rod. I didnt want to bugger up the rod with vice grips and that nut was pretty tight (perhaps tightened all the way in). Any suggestions?

[Just 6T9 CHGR]

The rod is knurled for the purpose of using pliers to hold it while turning the acorn nut....you wont damage it at all.

Do you have any threads showing?  IIRC when I had my "correct" master on I had about 2-3 threads showing.......they are all different though you have to adjust to your master

See here as well

http://www.dodgecharger.com/forum/index.php/topic,27395.0.html

[Mfr426]

Yep, I saw the knurl but had a hell of a time getting it to loosen. I'll try it again this week. The directions on the other post are much better than the ones in the FSM. They are way to basic and general.

Thanks

Mike

[is_it_EVER_done?]

15" of vacuum should be plenty to operate power brakes properly, so it sounds like you have either a bad booster, or a M/C with to big a bore.

Is there a difference in pedal effort between running and a shut off engine? Are you certain that the booster is actually getting vacuum?

Just to save you the time and trouble, adding a vacuum reservoir will not help anything as it can only store the amount of vacuum you already have (15"), so if it isn't enough now, a larger reserve won't change anything.

If you are certain that your booster is working and have access to any other M/C's, try a power drum brake one (1" bore), or an A-body disc unit that's 15/16th" bore.

[Mfr426]

One item that I neglected to mention is that years ago I put on A-body spindels from a 73 dart and did the swap from y 69 discs. I could not afford the calipers for the 69 so I had to do something. I did put on a new booster but referenced one for a 69 with power disc brakes. Dont know if that makes any difference here?

Thanks for the continued help...

[is_it_EVER_done?]

The conversion makes no difference. Did the stock system work before you changed over? After you changed the booster, did the brakes work properly?

What exactly do the brakes feel like. is the pedal hard but firm, or is it spongy. is the pedal high, or does it go close to the floor?

My guess is that you have a bad booster, but it's possible that if/when you changed the M/C that you got a big bore unit by accident.

[Mfr426]

The pedal is stiff and does not go to the floor. Christ, if I have to climb under that dash one more time for those 4 nuts I'm going to welll....welll...I'm going to have to do it...

[8WHEELER]

I just put a new booster on last year, the one before that lasted one year. The diaphragm was completely blown.

The rebuild shops these days can be good, or they turn out junk, they will admit to it. They told me
last year, they can go through 10 rebuilds, check them a week later, and only 7 out of the 10
are still working properly.

edit: OK you answered this while I was slowly typing.
Yes, How hard is you pedal? more often than not, if you loose your booster, your pedal gets harder.

Dan

[Mfr426]

OK...no laughing at this question...

I can go onto Advance Auto and they have boosters for my car (disc) for about $90.00. I think they were Cardone but am wondering if they are correct for my car??? I mean that from a functional as well as a visual standpoint...

[Just 6T9 CHGR]

What is the Cardone part #?  Thats what I have on my car....


Hold up.....looking at the Advance Auto (PartsAmerica.com) website?  Yep they show them but when you try to buy it they tell you there is no stock.........lets you go through the whole ordering process to get an email later on saying they are cancelling your order due to no stock from the mfg.........been there, done that 

[Mfr426]

I'm calling Adavance and AutoZone now. They can tell you if iit's in the system and are usually pretty accurate...

Can you actually test the booster while in the car?

[Mfr426]

Chris, yep...that's the Cardone part (5473603). I'm calling both places now...

Advance and AutoZone have the part listed but they cant order it. This is like deja vu all over again with my MC from a few months ago...

I just called National who was able to get me the correct M/C a few months ago. Unfortunately he just called me back after talking with all his suppliers and it is no longer available...

I'm hoping for a pushrod adjustment...

[GreenMachine]

I'm hoping for a pushrod adjustment...

Hey Mfr426, my brakes are about the same you are describing. They are firm and won't lock up no matter how hard you step on them and slowly comes to a stop. I accidently ordered a manual disc conversion kit instead of a power kit, so I had to buy a power booster separate from the parts store. After reading this post, I checked my rod and the nut is bottomed. I adjusted it out but haven't taken it for a test drive yet. I might drive it to work tomorrow, if so I'll let you know how it worked.

[is_it_EVER_done?]

If you have REASON to believe that it is the booster pushrod, go ahead and try an adjustment. I just hate to be the one to tell you that it isn't the problem, and messing with the adjustment will only require you to find the proper adjustment once you find the actual problem.

A misadjusted booster pushrod will either give you an extreme amount of pedal travel prior to brake engagement, or dragging brakes. By design, they don't have enough adjustment to cause the M/C piston to cause the brakes to lock on (like a line lock).  In any event, it can't cause the problems you are describing, nor cause the booster to fail.

If you have access to a vacuum pump, you can test the booster, or a fully equipped brake shop should be able to test it with 20+ inches of vacuum.

All the evidence indicates a bad booster (15" steady vacuum, no fluid leakage, high/hard pedal). If the booster is actually getting the vacuum (no stuck check valve), I would have to say that it's over 95% chance that it's a bad booster.

[Mfr426]

OK, let's assume it's a bad booster...what are my options to replace this? Sounds like the normal avenues (Advance, Napa, National) are all exhausted...

[8WHEELER]

Sorry I can't help. I was able to get two of them two years ago, one from Napa, and I went right
back, and he started looking and calling, no more available................ same thing with the brake
booster rebuild shop in Cal, they had one more on the shelf, they just did not get anymore in.

I have seen rusty units needing a rebuild, selling on ebay getting $300.00 plus bucks,
they are very hard to find even used at this point   

Dan

[Just 6T9 CHGR]

I got mine from CA back in January and was probably one of the last "available" at the time.

Booster Dewey does a great job rebuilding them.  Turn around might not be that long now because the season has started....

http://boosterdeweyexchange.com

[Mfr426]

I have not had time to check the piston travel (to try to figure out if the rod adjustment could have anything to do with it). However, now having time to digest the posts and the situation I would tend to agree that my booster is toast. Now, to try to find one...

[resq302]

I would say just send yours out to have it rebuilt since trying to find another one will be like hitting the lottery.  Next to zero chance in hell.

[Mfr426]

Just checked out Booster Dewey's page. It looks like $100 plus shipping to rebuild. That seems fair and I do have a core to send him. That's bascially what I was going to spend locally here (if I could have purchased one).

Thanks all. I think once I confirm that the rod is correct that will be my next move.

Mike R

[GreenMachine]

You can check your power booster by running the car then turn it off, then pull the check valve out to see if it's holding vacuum. The rod adjustment didn't help me. I'm thinking I'll take 'is_it_EVER_done?'s advice and change the master cylinder since my kit was for manual brakes.

[Mfr426]

Hey Green, I like that test...I'll try shortly...thanks...

Miike

[Mfr426]

OK...tested last night (fired up car, let it run for about 2 minutes and turned it off) and I had very good vacuum when I pulled the check valve from the the booster. If that's the test then mine passed with flying colors.

I have a hard time with master cylinder being bad since it's brand new (and I put it throug the NEW bench bleeding routine) and it pumped right up w ith no problems. It didnt leak either.

My only hope is that the rod is just out of adjustment. If only I had time to spend a few hours doing this work. It's amazing how work intereferes with all my fun....

[Mfr426]

Good news folks. I got a chance to do the test on the pedal that looks for activity in the front reservoir with 1/2" of pedal travel. My wife was in the car and I had her push 1/2" on the brakes. Nothing. I had her go a bit more...NOTHING. I had her go another 1/2" and bingo I see some bubbles in the front reservoir. I had her test that several times and it was very obvious that the pedal was not pushing in enough early enough (rod too short) to fully plunge the piston in the M/C.

So, I pulled off the M/C and attempted (once again) to adjust that acorn nut. I used vice grips (I know I HATE using vice grips) on the main rod and tried to unscrew the nut. No luck. It was pretty seized. So, I had to soak it in D40 and I ended up using a deep well, 3/8" deep well socket. That did the trick. I backed out the nut about 2 turns (cant be sure but it was a fair amount). I then proceeded to bench bleed the M/C since it was off the car (I am convinced however that the new bench bleed technique did work the first time). I bolted on the M/C and lines and thought I'd just try it without bleeding the rest of the system. Sure enough I had "touchy" brakes right away. I could get the fronts to lock up with some real pressure on them. Before I would have to stand on them to get any kind of good braking. I figure with further bleeding it should stop on a dime.

Here's the lesson...if you have brake problems before you begin replacing parts check that rod adjustment. My booster is fine and my M/C is fine. That blasted rod and nut were out of adjustment. Oddly the FSM only mentions it in passing and only covers it briefly. That's another oversight by the author in my opinion.

Anyway, thanks all for reading and offering advice. The vacuum test of the booster is great and the brake test (try 1/2" then more to see if you have reservoir activity (bubbles) was the final evidence that I had the problem narrowed down.

Mike R in Reading PA...
Braking better tonight!

[Just 6T9 CHGR]

Good news!

[is_it_EVER_done?]

Good news folks. I got a chance to do the test on the pedal that looks for activity in the front reservoir with 1/2" of pedal travel. My wife was in the car and I had her push 1/2" on the brakes. Nothing. I had her go a bit more...NOTHING. I had her go another 1/2" and bingo I see some bubbles in the front reservoir. I had her test that several times and it was very obvious that the pedal was not pushing in enough early enough (rod too short) to fully plunge the piston in the M/C.

So, I pulled off the M/C and attempted (once again) to adjust that acorn nut. I used vice grips (I know I HATE using vice grips) on the main rod and tried to unscrew the nut. No luck. It was pretty seized. So, I had to soak it in D40 and I ended up using a deep well, 3/8" deep well socket. That did the trick. I backed out the nut about 2 turns (cant be sure but it was a fair amount). I then proceeded to bench bleed the M/C since it was off the car (I am convinced however that the new bench bleed technique did work the first time). I bolted on the M/C and lines and thought I'd just try it without bleeding the rest of the system. Sure enough I had "touchy" brakes right away. I could get the fronts to lock up with some real pressure on them. Before I would have to stand on them to get any kind of good braking. I figure with further bleeding it should stop on a dime.

Here's the lesson...if you have brake problems before you begin replacing parts check that rod adjustment. My booster is fine and my M/C is fine. That blasted rod and nut were out of adjustment. Oddly the FSM only mentions it in passing and only covers it briefly. That's another oversight by the author in my opinion.

Anyway, thanks all for reading and offering advice. The vacuum test of the booster is great and the brake test (try 1/2" then more to see if you have reservoir activity (bubbles) was the final evidence that I had the problem narrowed down.

Mike R in Reading PA...
Braking better tonight!

Mfr426:  Though you believe you found your problem, if you would think about it in a logical/analytical way, you will realize that the 1/8th" or so of total adjustment in the acorn nut can't have a bearing on the M/C piston traveling far enough to actuate the brakes. Consider this -- The brake pedal can travel 4 to 5 inches and can push the M/C piston to the end of it's travel. Unless your brake pedal is going to the floor, that 1/8th inch just doesn't make a relevant difference as far as M/C piston travel is concerned.

You bench bled the M/C (you say you saw bubbles in the front reservoir which is air in the system), yet you attribute better brakes to the booster pushrod adjustment, which is the least important adjustment in the entire brake system. That's all fine if that is what you want to believe, but you are doing a great disservice to others by suggesting that they play with the adjustment if they are having brake problems.

You state that the FSM doesn't cover this enough, but it does because it gives you a measurement, and there is so much leeway in actual operation that there is just nothing more that they can say about it.

The booster to M/C pushrod adjustment is there to eliminate excessive play (and the subsequent reduction in the vacuum supply of the booster) in the brake pedal before the M/C supplies pressure to the brakes. One of the tell tale signs of too much free play is excessive movement of the brake pedal and an audible "WHOSH" sound as the booster vents vacuum before the pedal gets firm due to engaging the M/C piston.

A too tight adjustment manifests itself in a brake pedal with "instant on" or "touchy" type brakes, instead of good modulation. By design there is not supposed to be enough adjustment to push the M/C piston past the reservoir port (which would cause the brakes to drag), but it is possible on some systems.

If the symptoms you have been posting over the last couple of months are accurate, and IF adjusting your booster pushrod helps or fixes the problems you have been having, I caution you that you have a problem somewhere else that WILL re-manifest itself, possibly while attempting to brake on the freeway with your family in the car!

My suggestion is that you take the car to a brake shop (not one of the $99.00 pad replacement places). Let them go over everything before you hit the road with wife/family in tow.

If this site had a saveable tech section, I would take the time to write out a flow sheet of possible problems/tests/remedies, but it's just to extensive to do for a single thread. Just be aware that our ancient brake systems can suffer from a myriad of maladies that are not covered in standard brake troubleshooting methods (rust/gunk in the safety valve, proportioning valve, M/C or brake pistons. Residual valves which should or should not be there or redundant/doubled. M/C bore size to brake piston sizes, and a host of others, but the entire brake system pales in comparison to the complexity of the brake booster.

Keep in mind that a booster is supposed to deliver the same assist through a constantly varying vacuum supply. In other words, the brakes must work and feel the same in idling traffic, as well as either high RPM acceleration (very low vacuum), or the reverse of which is a suddenly closed throttle at high RPM which will deliver vacuum close to absolute. In order to do this, a booster is a complex unit of valving, spring pressures, vents, etc.

The only reason I bring this up is that if an otherwise working/sealed brake system (no leaks, contamination, air in the system, etc.) becomes hard to operate due to a high hard brake pedal, it is NEVER due to the booster pushrod becoming out of adjustment.

I honestly don't intend any meanness or disrespect to you, but because I can envision a hundred people going out and messing with their booster pushrod adjustment due to your statements, I feel it's necessary to stop them if I can.

[Just 6T9 CHGR]

Great points!   Although Mike's pushrod never became out of adjustment.   He put his new booster on and never adjusted it correctly.

The FSM doesnt give a measurement per say for the disc brale Bendix booster.  The measurement pictured is for the Midland Ross drum brake booster.  The Bendix booster only gives a procedure for checking proper booster rod adjustment by viewing the eruption of brake fluid in the master when the pedal is pushed....

[Mfr426]

Thanks for the trip to the woodshed Is it ever done but I respectfully disagree with your summary. You mention that the 1/8" difference could not make the difference in the braking performance that I have experienced. As an example of how that CAN effect the braking, we had a similar (yet completely opposite issue) happen with my buddies 68 RT that we did motor work on a few years ago. His brakes would completely lock up after a few minutes of driving after we removed his M/C and booster. We had changed nothing in his brake system other than to remove his M/C and booster so we could paint his engine bay. After thoroughly reviewing his FSM he found the comments about the push rod. We adjusted the acorn nut in a few turns (would not swear it was in but that makes sense) and his issue went away. We did nothing other than that adjustment.

Also, even though you can get 4-5" of travel out of the pedal you dint get that out of the piston. There is a pivot point on the brake mechanism that changes the actual distance that the rod can travel. In other words, if you push the brakes down 5" you would not see the rod come out 5". It's physically impossible as the M/C piston sweep is no where near that long of a throw. That also is why an 1/8" adjustment either way can make such a large difference in brake performance.

With regard the M/C and bench bleeding, I have done that multiple times with the correct fittings running right into each reservoir on more than one M/C on my car and that has never made a difference in the end result of the brake performance(in this car and issue at least). While installing a M/C without bleeding CAN have a huge effect on your braking (in a negative way), I have always bled the M/C prior to installing. The bench bleed directions that were included with the rebuilt M/C accomplished the same end result as the old M/C in the vise technique. I've literally had that M/C off 5 times and bench bled it each time I went to reinstall with no improvement on the braking. The lack of bench bleed and proper bleeding of the system was my first order of business when I was attempting to troubleshoot this issue months ago.

I think time will tell if I actually did find the culprit. I'm "reasonably" confident that I did since my brakes are touchier than ever and I still have to bleed the rest of the system.

Also, I don't take the family out (no car seat anchors and the old seat belt technique does not give me the warm and fuzzies with my kids).

Mike R

[Ghoste]

Whatever made the differnce, I'm just glad to hear you can stop your car again Mike.

[Mfr426]

Thanks Ghoste, I'm glad too!