ok changed the brakes on a 99 plymouth breeze, upon bleeding them and putting everything back together, the brake pedal goes to the floor, first assumption was air in the lines, rebled the system, and tried again to no avail. Next step was to change the master cylinder, only because we already had an old wrecked one, so it was a no charge chance to fix the issue, still the same thing, if car is off, brake pedal pumps up and holds stiff, however with car running, pedal goes right back to the floor, any thoughts here, im thinking maybe a bad wheel cylinder on the drums in the back, or a proportioning valve, just wondering what you guys think?

 

Make sure you bench bled the M.C. and re-bleed. and check hoses and wheel cylinders for and leaks. You might wanna try asking this question in the appropriate section. They might have better input.

 

pedal tight with engine off, goes to the floor with engine running....
i'd say its the brake vacuum booster leaking.
i'm looking for a link on how its supposed to work.

http://auto.howstuffworks.com/auto-p...wer-brake1.htm

 

brake booster. check that engine has proper vaccum (15 in hg), then if so replace booster

 

POWER BRAKE BOOSTER
All vehicles use a 205 mm tandem diaphragm power brake vacuum booster. The power brake vacuum booster though, may be unique for the type of brake system the vehicle is equipped with. For this reason, if the power brake vacuum booster requires replacement, be sure it is replaced with the correct part for the type of brake system that the vehicle is equipped with.

The power brake booster can be identified by the tag attached to the body of the booster assembly Power Brake Booster Identification This tag contains the following information: The production part number of the power booster assembly, the date it was built, who manufactured it, and brake sales code.

NOTE: The power brake booster assembly is not a repairable part and must be replaced as a complete unit if it is found to be faulty in any way. The power booster vacuum check valve is not repairable but can be replaced as an assembly

The power brake booster reduces amount of force required by the driver to obtain the necessary hydraulic pressure to stop vehicle.

The power brake booster is vacuum operated. The vacuum is supplied from the intake manifold on the engine through the power brake booster check valve Power Brake Booster Identification

As the brake pedal is depressed, the power boosters input rod moves forward Power Brake Booster Assembly This opens and closes valves in the power booster, allowing atmospheric pressure to enter on one side of a diaphragm. Engine vacuum is always present on the other side. This difference in pressure forces the output rod of the power booster Power Brake Booster Assembly out against the primary piston of the master cylinder. As the pistons in the master cylinder move forward this creates the hydraulic pressure in the brake system

Different systems and engine combinations require different vacuum hose routings.

The power brake vacuum booster assembly mounts on the engine side of the dash panel. It is connected to the brake pedal by the input push rod Power Brake Booster Assembly A vacuum line connects the power booster to the intake manifold. The master cylinder is bolted to the front of the power brake vacuum booster assembly.

 

hey motorbreath, where you been for the last 2 years?

i've never done much with boosters, so i don't know, but reading the internet articles - they mostly say that a bad booster will cause a hard pedal (no boost), rather than a pedal that collapses.

 

hey vaughan! been busy with chrysler school, finishin up my last quarter. also restorin a plymouth duster right now, so been pretty busy. tryin to catch back up with yall though!

 

haha, just re-read the old threads - same old stuff. plenum, transmissions, idle, cruise surging, steering, speed sensor, heater core, cracked dash.

are you working at a dealer ?

.. i guess i need to quit jacking this thread.