47-59 Power Brake Oddity

[MiraclePieCo]

My panel has a dual diaphragm power brake booster. Works great BUT...

Whenever I turn off the engine with my foot on the brake, the pedal remains down about 1/2-inch which often leaves my brake lights on. During normal driving the pedal always returns fully. It has an auxiliary return spring - I could make it stronger, but I'm reluctant because I don't want it adding to my pedal pressure.

Any suggestions?

[mr48chev]

I'd say that you are going to have to switch to just enough auxiliary spring to get it back. It would take a pretty stout spring for you to even notice the difference though.

[dsraven]

simple.
dont turn the engine off with your foot on the pedal.
haha
but really, it sounds like you have an internal vacuum leak in the booster or a leaky check valve or poor grommet seal at the supply hose on the booster. also check for sticky pedal pivot or sticky or "rubbing on something" linkage problem, a booster to master cyl pushrod that possibly needs some adjustment (not enough clearance), binding linkage, a bad vacuum check valve or possibly a leak in your brake booster. some boosters have a return spring inside them that assists getting the pedal back where it should be. possibly that is weak or broken. if you installed a pedal return spring would that fix the problem? with a frame mounted booster there is a bit of linkage involved from the pedal to the master, usually. maybe start be checking all those pivots and bellcranks etc to ensure everything moves super freely. a bit of a tight spot in each joint, added all up, could be a lot of friction and possibly too much for the return spring to overcome.
try this

-with engine off, pump the brakes a few times to deplete the system of any vacuum in the booster. now, while holding brake pressure, start the engine. the pedal should drop slightly, like 1/4 to 1/2 an inch.
-next start the engine to build vacuum in the booster. then turn the engine off and do a couple of brake applications. the first should feel like normal and then consecutive applications should get firmer and firmer until they feel like manual brakes.
-now start the engine and let the vacuum build to normal. a minute of running should do it. step on the brakes and shut the engine off. the pedal should stay at the same height. if not check for a vacuum leak or possibly an internal leak in the booster or vacuum supply hose check valve. a vacuum gauge installed between the check valve and the booster would show what the booster is doing with the vacuum that should be stored inside the booster. you should have approx 18 in of vac on a normal engine when at operating temp.
just found this from roadkill customs. maybe help you?

https://m.roadkillcustoms.com/test-power-brake-booster/

[dsraven]

another "diagnose your booster" link. maybe help you.
also check your vacuum hose and port on the engine side of things.

https://axleaddict.com/auto-repair/B...-Booster-Check

[MiraclePieCo]

Originally Posted by dsraven

simple.
dont turn the engine off with your foot on the pedal.
haha

It's a habit - I can't help it!

[mr48chev]

I've had the same habit for 60 years. I always have my foot on the brake on newer automatic trans rigs when I start them too. I got in that habit in auto shop in high school when a car I started jumped because it was in gear.

[dsraven]

nowadays you would be hard pressed to find a high school kid that knows the difference between screwdrivers let alone working on a car. if you asked a kid for a phillips screwdriver they would look it up on google. sad but true for a lot of youngsters. then there is a FEW at the other end of the spectrum who could probably program a tune for your car.

[G&R's57GMC]

If all the above check out good, you may want to change the stop light switch to a hydraulic one plumbed into the brake system.
Just a thought .

[1project2many]

This problem intrigues me. I expect you will find the issue is in the booster. I'm curious though. Does the pedal have plenty of assist if you press it after the engine is off? How about if you wait for a period of time? An hour? Overnight?

What happens if you shut the engine off with the brake pedal released then press and release the pedal shortly after the engine is off? Does the pedal return or hang up on release?

Was there an OE application for this booster and master?

[MiraclePieCo]

Thanks for the suggestions gentlemen. I'll perform the recommended tests and let you know.

BTW, It's a new POL kit, dual booster. After the engine is shut off (no vacuum) the pedal releases fully. There is no binding in the linkage and the pushrod has been properly adjusted with the special tool.

[MiraclePieCo]

Okay, looks like I fixed it - or at least covered it up. I just shortened my pedal return spring to make it apply more pressure. Worked perfectly during testing.

Originally Posted by mr48chev

I'd say that you are going to have to switch to just enough auxiliary spring to get it back. It would take a pretty stout spring for you to even notice the difference though.

You were right; it might exert 2 lbs of force. I can't even tell the difference.