When I am stoped at a light or in traffic, the brake pedal will sink on me to the floor. If i sit for too long the brakes will fail completely and the truck will start to roll. I have bled the brakes it is full on fluid.

Any thoughts yall, please....

 

Bump for help.

 

Yes it sounds like the mastercylinder is leaking by the seals in side you mite see moisture were it bolts to the power booster.

if you can pump it up again when it goes to the floor
it should be the masterclyinder