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

[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!