Brake Pedal lazy return

[Ron H]

As I'm back to this fix-it project my '63 Plymouth had a power-disk brake system installed 3 years ago. A lot of reworking was necessary as the pedal height was all goofed up among a few cosmetic issues I've fixed. The pedal remains a problem after getting it adjusted so it was a foot brake and not a hand brake (ridiculously high pedal setting). This involved shortening the pushrod and making a new-more durable eyebolt. The problem, now more so since motor beef up including mildly hotter cam, is pedal won't return fully and seems to suck down after extended driving to the point brake lights go on. I did a temporary fix installing a return spring that helped; but evident a stronger spring would be needed. Not a best "fix" but many seem to have gone this route so not totally averse to it. My guess is the cam is an issue but also heard of vac boosters. If anyone has encountered and solved sure like to hear. Thanks

 

[Yatzee]

As I'm back to this fix-it project my '63 Plymouth had a power-disk brake system installed 3 years ago. A lot of reworking was necessary as the pedal height was all goofed up among a few cosmetic issues I've fixed. The pedal remains a problem after getting it adjusted so it was a foot brake and not a hand brake (ridiculously high pedal setting). This involved shortening the pushrod and making a new-more durable eyebolt. The problem, now more so since motor beef up including mildly hotter cam, is pedal won't return fully and seems to suck down after extended driving to the point brake lights go on. I did a temporary fix installing a return spring that helped; but evident a stronger spring would be needed. Not a best "fix" but many seem to have gone this route so not totally averse to it. My guess is the cam is an issue but also heard of vac boosters. If anyone has encountered and solved sure like to hear. Thanks

I believe shortening the push rod was not the best of choices. As you should be aware, the master has a built in spring that sends the internal spool back to it's resting position. A radical cam only lessens vacuum and you would have little to no assist except at higher rpm's and during deceleration when vacuum goes high. First and formally you have to make sure you have the right amount of stroke on the master, not partial. Next you have to ensure the proper pedal ratio is being met for the type of system you finally came up with. Any where on this page will verify your pedal ratio:

https://www.google.com/search?q=brake+pedal+ratio&ie=utf-8&oe=utf-8&client=firefox-b

And finally you need a minimum of 17 to 18 inches of vacuum for reliable brakes under all driving conditions with vacuum assist. I had the exact same problem as you and instead of beating a dead dog to death I bit the bullet and installed hydra-boost which requires no vacuum but does require an existing power steering pump. If this route is not to your liking you can install a vacuum pump and hope for the best. Used hydra-boost units can usually be found at auto recyclers' for $50 to $150. But regardless, the pedal should return to it's upmost position and the spool in the master should totally return to rest position with the car running or shut off. Springs on brake pedals were used mainly to keep the arm against it's "stop" bumper so it didn't thump on rough roads but I guess a heavy one would return the arm completely. The question would be if the master spool is also returning.

 

[steve340]

I have shortened booster pushrods before. They can be shortened but you have to be careful you do not create binding or another problem as Yatzee states in his post. You need to make sure there is a small amount of free play in the pushrod between the booster and the brake pedal. There is an air inlet valve in the back of the booster that may not be fully closed. If this valve is leaking this will allow engine vacuum to operate on the booster at all times.
Sucking down of the pedal has nothing to do with the engine it is only supplying vacuum at this point.
You could also have a fault in the booster unit.

 

[Ron H]

This is a head scratcher. Did an awful lot of futzing with the rod adjustment and it's very close between brake engagement or free, just a turn or so on the threaded rod is it. Brakes work great. Following what you say there's more to be aware of messing with this. It's strange as the pedal seeps down and it's easy to just put a foot under the pedal and bring it back to full return don't feel any binding; but a resistance doing it. I suppose air or vacuum? It works fine (comes back to full return) then it mysteriously seeps back down after a while and often the case when first starting the car. Guess I'll have to stare at again for a while to get a visual in my head with what you both have talked about here. Thanks much.

 

[steve340]

You could try disconnecting the pedal and get it up out of the way and make a mark you can reference too, perhaps tape line or such like. A bit of welding wire under a screw lined up with the mark would be good. Nothing that will interfere with movement.
Start the engine and see if the push rod gets sucked down.
If it does I would be heading towards a fault in the booster.
Be aware when you start the motor the vacuum will come on the booster diaphragm and it will settle a bit but from there should not move.
Have it jacked up so you can test if the front brakes start coming on.