Brake pedal sometime returns, sometime not

[Kimodc]

So here is my situation and I am not sure what to do next. I have a 1969 Sport Satellite, originally manual drums, now factory 1970 b body 11” discs up front and factory 10” drums on rear. I installed a Pirate Jack’s power booster and drum/disc master cylinder and a correct proportioning valve. At first, the pedal would sink to the floor. I could never get the rear circuit to bench bleed correctly. I replaced with a Raybestos MC and brakes feel great and I can easily lock up the 245 wheels up front. The problem is, every so often, the brake pedal will stay depressed and not spring back like it should. Next 3 traffic lights it works great, then decides to stay depressed. Linkages are all clean and lubed, no brake fluid leaks anywhere. The system has been bled numerous times. I took it to a local speed shop, they had it for two weeks and were stumped as well. Any ideas? Where do I look next?

 

[mrmolding]

When I converted from manual to power I had a little play with pedal not returning. There is a brake pedal return spring for power brakes. Probably not all of your issue, but here is a thread with a pic of when I installed mine.
Whats everyone working on?

 

[steve340]

When the pedal stays down are the brakes lightly applied?
The rod between the brake pedal and the booster could need to be a little shorter or a spring is required to bring the pedal right back.
There is a little air valve on the *** end of the booster that could either be faulty or being held a little open.
This valve allows air in to the back chamber of the booster and therefore allowing the manifold vacuum to suck the booster diaphragm forward.

 

[Kimodc]

I was unaware that power brakes used a return spring. Mrmoulding, where
did you find a spring? It sounds like a spring might be the answer.
Yes, the brakes do apply slightly when the pedal stays down. I am not sure how a shorter rod between the pedal and the booster will help, I would think it would just cause the pedal to sit a little lower. The air valve assembly sounds like it may be a culprit. It is internal to the booster, and if so, is booster replacement the solution?
Thank you for the suggestions, I will try a spring first and see if that makes the pedal return predictably every time. Thanks again! I appreciate the help

 

[mrmolding]

I was unaware that power brakes used a return spring. Mrmoulding, where
did you find a spring? It sounds like a spring might be the answer.
Yes, the brakes do apply slightly when the pedal stays down. I am not sure how a shorter rod between the pedal and the booster will help, I would think it would just cause the pedal to sit a little lower. The air valve assembly sounds like it may be a culprit. It is internal to the booster, and if so, is booster replacement the solution?
Thank you for the suggestions, I will try a spring first and see if that makes the pedal return predictably every time. Thanks again! I appreciate the help

I got it from this eBay seller. I have no idea if he has any more and it was not cheap.

moparmendyb-bodyparts

 

[Kimodc]

Thanks Mrmolding, he does have them for sale, but WOW!!! you weren’t kidding on pricey!!! I think I will try my local Ace Hardware first and see what I can find, don’t tell Galen Govier.

 

[steve340]

Some boosters have a threaded adjustment rod you can shorten. Sounds like yours are fixed length so try the spring to pull the brake pedal fully back.
If the air valve is faulty the booster would need repair or replacement.

 

[mrmolding]

Thanks Mrmolding, he does have them for sale, but WOW!!! you weren’t kidding on pricey!!! I think I will try my local Ace Hardware first and see what I can find, don’t tell Galen Govier.

I struggled with price for a few days, looked for something similar, and then gave up and bought one. Just enough tension and works great. Opportunity cost I guess! He knows what he has!!!

 

[Kimodc]

I struggled with price for a few days, looked for something similar, and then gave up and bought one. Just enough tension and works great. Opportunity cost I guess! He knows what he has!!!

I hear what you’re saying……wouldn’t be the first time I tripped over a dollar to pick up a dime!

 

[Kimodc]

Some boosters have a threaded adjustment rod you can shorten. Sounds like yours are fixed length so try the spring to pull the brake pedal fully back.
If the air valve is faulty the booster would need repair or replacement.

Thanks Steve, I can’t remember for certain, but I think it is a fixed length. I will try the spring first

 

[QuickBpBp]

I got it from this eBay seller. I have no idea if he has any more and it was not cheap.

moparmendyb-bodyparts

@moparmendy is a member here also...

 

[Kimodc]

I put a spring in and no difference. I ordered another brake booster from the Dr……we will see if that makes a difference. Not sure what else to do. I did unhook the vacuum line from the booster and the brake pedal did not stay engaged, but I also couldn’t get it to go down as far. I will report back when I get the new booster installed

 

[steve340]

Good luck hope you get a good result.
Do you have the Bendix copy style booster?

 

[gtxdana]

Is that the aftermarket 9" dual diaphragm booster that everyone sells for all makes and models? MBM in China makes them and I've had several, not on a Mopar but it's the same part, that would intermittently stick just enough to keep the brake lights on and but not enough to cause a brake drag. It's a problem, but you'll get the "we never heard that before" response.

It's internal in the booster. Some thought the chrome was too thick causing the seal to be too tight on the piston causing the pedal to stick but I did not see that.

I installed a new brake pedal spring. That was no help. I installed an extra spring to pull the pedal all the way back up which worked. I then found that if you adjust the booster pushrod at the pedal to preload the booster a slight amount, that also cured the problem. I did reinstall the extra spring as well.