98 Pontiac Transport - Brake Problems

[issa1864]

I have a 98 Pontiac Transport. We have changed all 4 brake pads, bled them and I am still having a problem.

The brake pedal when the engine is off has full brake, however, once the engine is turned the brake pedal seeps to the floor and will not hold pressure, we do not see any leaking. Please let me know what the cause of this....Is it the master cylinder or extorsion valve.

[LMP]

Originally Posted by issa1864

...changed all 4 brake pads, bled them and I am still having a problem.
.

So..what was the problem before you changed the brake pads?..
Is that disk on all four or drums at the rear end?

As a starter: when engine does not run, you do not have vacuum left in the system and that is why the pedal is hard; as soon as engine runs you have brake assistance and that is why the pedal is softer...but of course it should not go to the floor.
If is was like that before you replaced brakes and this is the reason you changed brakes , but to no avail, then I would seriously consider master brake cylinder is shot. Got that on son's Lumina last summer.

[issa1864]

Thats what it was. Thanks.