Sinking brake pedal

[boschbabe]

When we got my daughter's '93 auto the brake pedal would slowly creep to the floor like it had a leak but there wasn't one. My son replaced the master cylinder while I was out of town. The pedal was lower than it should have been but they worked okay. Now they are exactly the way they were with the last m/c. If the m/c is leaking internally, where is the fluid going? The reservoir stays full. The replacement m/c is a rebuilt unit from Advanceauto.

[Briano1234]

Master Cylinders can leak internally as the seals don't push the fluid correctly and the fluid escapes past the seal.
Masters can leak at the piston end and drip into the brake booster.

If the master wasn't bench bleed then you may have residual air in it.

Use a Mighty vac on all the wheel cylinders to bleed them.

I have gotten 2 masters bad from the same place. So it could be the master isn't right.
It could also be that the main brake line to the booster is leaking or the vac line may be cracked.

Now with no engine running if you pump the brakes 3 times, then hold the pressure does it bleed off or sink?
Did your son adjust the free play on the pedal after they replaced the master?

[boschbabe]

Son did bench bleed. Did not adjust pushrod. Don't think master is leaking into booster. Reservoir is full. Could the master leak back into the reservoir? Brake goes to floor with or without boost (takes about 30-40 sec).

[Briano1234]

Son did bench bleed. Did not adjust pushrod. Don't think master is leaking into booster. Reservoir is full. Could the master leak back into the reservoir? Brake goes to floor with or without boost (takes about 30-40 sec).

Yes, go fetch a new one, and then make a fuss about all the brake fluid that you have had to use and see if they will give you a new quart. I did and they did.