Technical Sticking brake pedal wont return

I've spent the better part of today replacing my dual diaphragm brake booster but I still have a recurring and frustrating intermittent sticking brake pedal problem.

My setup is: 1950 Chevy Fleetline, disc front/drum rears, dual under floor master with 7" dual diaphragm booster, 2 and 10 lb residual valves and a combination valve. Rears are 1950. I have no leaks and as mentioned the booster was replaced today as the old one pushed the front seal out and was leaking vacuum. Vacuum at idle is 18lbs and pretty steady.

The problem is I can press the brake pedal quickly and it activates the brakes as it should, the pedal pops back up and all is good. In fact I can press and release the pedal rapidly reversing down my driveway with no issues. However when I push the pedal down and hold it down, driving on a level street, the pedal stays in the down position with the brakes locked until I reach down and pull the pedal back up.

The pedal moves freely through the the toe board without rubbing. I thought the bottom of the pedal might be rubbing on the chassis as I extended it to change the pedal ratio. MP Brakes had a recommended pedal ratio for power brakes- it escapes me now, maybe 5:1? Anyway it clears now.

I've read that the residual valves can be problematic and prone to failure often. Their quality is dubious I guess and I've heard of more than one person replacing them multiple times. It does seem like it could be a valving problem since it switches from operating just fine to locking and not releasing.

I'm really at a loss as to what could be causing it to stick when held down, knowing that the return spring is strong, the booster is new, vacuum is good etc.

Another suggestion (which I haven't had time to try) is that the booster is not fully retracting from the master cylinder, but if that adjustment was off I think the problem would be constant not intermitant.

As usual any advice or ideas is much appreciated!

 

You do have a return spring on you pedal, Right? HRP

 

Yes, there's the stock pedal return spring in place, which is plenty hefty.

 

have you checked the pedal pivot point for grease ??? if its dry it can hang up and being under the floor it sees dirt and water , doesn't take much rust to lock things up .

 

I replaced the master cylinder in my olds wagon for the same problem. Still had the same problem , even with a spring.Turned out it was a crappy rebuilt master cylinder, so for $5. more I bought a new one and the problem disappeared. Bruce.

 

Thanks for the suggestions so far. The pivot point doesn't stick as is well lubricated. I guess the thing that throws me off is that the pedal releases just fine when applied quickly and while I'm reversing down my driveway. So the system works as it should in that situation. However when it levels out, the pedal sticks— and both situation are consistent.

When its in the stuck position does it mean the master is not fully releasing?

 

I would say YES. And/Or there's a problem with the new booster

 

Does your pivot point consist of a through bolt and nut setup? I can get the brake pedal on the '35 to stick if I crank it down a little too tight.

 

It's funny you mention that— I do have an eccentric through bolt, nut and cotter pin. I went out in the garage at lunchtime and backed off that nut as I had tightened it down to get the cotter pin through the bolt. I didn't have time to do a lot of testing, but from my quick lap around the block no sticking so far! I think that simple fix could be the problem. Thanks again for all of the suggestions!