68 c10 brake problems

[Gav2606]

Hi all. Ive got 68 c10 with s10 front discs and camaro rear axle with druns. I got in the truck this week and the brake pedal went to the floor. It was fine when i parked it up last time. Fluid level is good. It had dual master cylinder with a remote booster for the front brakes. Ive changed the master cylinder and no change. If i pump the pedal a few times a get a good pedal but leave it 3 seconds and then press it and goes to the floor again..i pumped them 40 times and the fluid level didnt drop and no visable leaks anywere. Could this be air in the system if the old master cylinder failed or any other ideas? The strange thing is dual system master cylinder i thought i would only loose front or rear not both. The brakes do work just right on the floor..

[firedemon]

sounds like air in the system

[Gav2606]

[Gav2606]

The block that the pipes go into. Is that a propertning valve. There is one hole with a bolt in it and its totally dry with no fluid coming out..is that correct? Also is the wire on the bottom for the brake warning light?

[ncpetersc]

Originally Posted by Gav2606

The block that the pipes go into. Is that a propertning valve. There is one hole with a bolt in it and its totally dry with no fluid coming out..is that correct? Also is the wire on the bottom for the brake warning light?

That is the proportioning valve, the wire is for the break warning light, I have no idea if the "dry hole" is correct.

[weq92f]

Wow, I've never even seen a remote booster before.

The pumping action you describe indicates a failing MC plunger. What if you pump up the brake pedal and keep the pressure on it. Does it slowly find its way to the floor while you hold the pressure on the pedal?

Air in the lines would make for a spongy, mushy pedal because that air will compress where hydraulic fluid without any air will not.

-klb