Brake problem

[mrnapolean1]

I have bled all four wheels and I get a steady stream of brake fluid thru the bleeders.

I have great pedal when the truck is in Park and running but shortly after I start driving I lose pedal and the Brake light comes on. It will stop but the pedal goes to the floor.

If I park and pump it up again, I get my pedal back.

What would be causing this?

 

[96Z71ECSB]

Is the truck running when you pump the brakes up?
Pump them up and keep constant pressure on the pedal.
If the pedal slowly goes to the floor, you have a bad master cylinder.

If the pedal stays firm, then you need to continue bleeding the brakes.
GMT400's are notorious for brake bleeding problems. Especially the '95 up 4 wheel anti-lock trucks.

To bleed the '95 up trucks, it's best to unplug the electrical plugs at the anti-lock unit. Unplugging it sets it into bypass mode.
Last resort would be to hook up a scan tool to cycle the anti-lock unit so it will bleed the trapped air.

Good luck!

 

[mrnapolean1]

During the bleed and pumping of the brakes the truck is not running. Its when I start it and drive ie the pedal goes to the floor.

 

[96Z71ECSB]

It shouldn't matter if the truck is running or not to check the master cylinder.
Pump the brakes up, once they are pumped up, keep constant pressure on the pedal.
If the pedal slowly goes to the floor, then the master cylinder is the culprit.

If it stays firm, then continue bleeding the system.
It may take quite a few bleeding sessions to get them bled properly.

 

[Eskimomann209]

My bets on that master