Brake Pedal Goes to Floor

[Bud]

Hi –

I have a 2002 Buick Rendezvous with a somewhat mushy brake pedal. While the brakes themselves seem to work fine, if I continually apply pressure to the pedal, it will sink to the floor. I recently did a complete brake job and replaced all four rotors and pads (it happened before that too), as well as the master cylinder. While I did bleed each, I did not do a complete flush of the entire system, which has not been done for the life of the car. Can old, dirty brake fluid in the lines cause this pedal sinking, or should I be looking elsewhere for a vacuum leak or a booster problem. Any advice on how to test a booster, as most point me in that direction? As stated, the brake pedal (and braking behavior) seems normal with the exception of the “going to the floor” issue. Is this “normal” for an old car?

Thanks,
Bud

Topic

[Ingvar]

No. It;s either master or you have rubber parts of hydraulic lines swell under pressure, resulting in pressure loss and pedal sink. GM products are known for that. Booster has none to do with this.
That’s if there are no leaks and fluid level stays steady.

[David]

While I’m not a mechanic, but it sounds like 3 possible things. With a new brake job its very possible to have air in the lines, that might cause it. Swelling in the rubber lines as suggested is another one. Usually though its the master cylinder.

Brake Booster is for assisting the driver and using less force at the pedal to achieve high pressure at the calipers/slave cylinders.

[dan]

typically when your pedal slowly sinks too the floor with constant pressure applied, it is usually the blame of the master cylinder, as pressure is applied the warn out rubber seals on the pistons inside the master cylinder allow pressurized break fluid too pass buy them slowly causing your pedal to slowly sink to the floor, air in the master cylinder can also give a spongy feel.

a bad break booster would cause your pedal to be very stiff requiring more effort for adequate braking on the drivers part, or the break booster seems to “lag.” in attempt to help breaking making the pedal feel way too stiff then slowly the break booster helps apply the breaks, but it won’t sink to the floor because of it.

I would make sure you have properly bled the master cylinder first, getting all the air out of the master cylinder, then once you have done that make sure you have bled the entire system, starting from the wheel furthest from the master cylinder typically the passenger rear, and then working your way up.

check this out

system

bleeding master cylinder

[Author]

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