master cylinder question

[Gary S.]

I have always been under the impression that when a master cylinder fails, the pedal goes to the floor rather quickly, with little or no braking, but can be pumped up to stop the car. On my car, the brakes stop the car just fine, but when you are sitting at a stop light the pedal slowly but steadily goes to the floor. Pumping them up one time brings the pedal back up. I have checked all four calipers and there isn't any leakage or fluid anywhere. MC level stays constant. All the bleeders are snug and not leaking. It has to be the MC but this only started after replacing rear rotors and flushing and filling the MC, then bleeding the entire system. Kind of odd timing don't you think?

Gary

[Bob R.]

Re: master cylinder question

It sounds like you must have air in the system. Under pressure from the brake pedal the air will compress causing the pedal to go down. Try bleeding again. Bleed all four wheels starting with the wheel farthest from the master cylinder.

[Dick W.]

Re: master cylinder question

Gary, this normally happens when the master cylinder starts bypassing. A slow bypass that allows the pedal, over time, to creep toward the floor. If you are not losing fluid, nor do you have signs of leakaage, this would be my prime suspect.

[John H.]

Re: master cylinder question

When the pedal slowly goes to the floor with a single application and no visible leaks, it's the master cylinder; time for a rebuild.

[Gary S.]

Re: master cylinder question

Thats for the responses - learn something new everyday.
Gary

[Eugene B.]

Re: master cylinder question

Gary,
What the guys described is somewhat typical of what happens when a brake job is performed with an old master cylinder (functioning just fine) in place.

When you bleed the brake system, you force the master cylinder piston into an area of the bore where it hasn't been in years. This area of the bore has all sorts of accumulated crud that scores the seals on the master cylinder rubber cups.

This is one reason that folks prefer pressure bleeding over pedal bleeding.

If the bore is in reasonably good shape, you can probably get by with honing the master cylinder instead of having it sleeved.

Good luck.

Regards,
Gene

[Gary S.]

Re: master cylinder question

Thanks Gene.