brake pedal travel

[diggerdpilot]

I have a buddy that has a 99 c1500, with a brake pedal that will function, but drop to the floor only occasionally. He says it feels like the anti lock brakes are firing, but the anti lock idiiot light doesnt come on. The master cyl. has been replaced, along with the fluid resevoir gasket.

Anyone have any problems similar? I am thinking a proportioning valve, or the ABS system, but I am not in real known turf on a Chevy.

Warped rotors have been brought up, but there is no brake chatter in the pedal, just it going to the floor in hard braking.

Thanks for whatever help we can get....

digger

[Thunderbolt]

Warped rotors wouldn't cause the pedal to drop, But if the rotors have been turned at or past there minimum spec they don't dissipate enough heat and can cause this.I f the brakes get to hot it can actually boli the brake fluid in the caliper and the pedal will fade. Did the truck just start doing this after replacing the mastercylinder or is this why it was replaced. Usually antilock problems are that they just don't work and that is the only problem I have ran across. That does not mean it can't be that.Make sure there are no leaks in the lines or wheel cylinders. Did you bench bleed the master cylinder before installing it ??

[diggerdpilot]

I am guessing the master cyl was bench bled, the installer was a shop mech. for our fire dept we work for. He doesnt really say it is spongy, we have bled the lines ourselves with good results.

It almost seems there is a "blow point" in the master cyl. that will give at a certain point of travel. We have talked about getting a bad MC from NAPA, but who knows.
Could it be a proportioning valve?

I am leaning towards replacing the MC, but still would like some opinions.

We checked the rotors tonight, just free spinning them, and they felt OK, just a tiny bit warped, but nothing I would write home about.
Then again, I am just a dumb Firefighter, not a tech....

Dig

[diggerdpilot]

I also just found out it does it at high and low speeds, so I am guessing the boiling of the fluid is out?

[Thunderbolt]

I have never seen a proportioning valve go bad. My guess is a bad master cylinder also.