Sometimes when at low RPMs my brakes will go out, usually right when i start my truck up, but sometimes after its been running too. It doesnt happen all the time either. I never know when its gonna happen, Hasnt happend at speed yet.... I bled them yesterday and i think i did a good job.. maybe not... I heard that the proportioning valve either WORKS or DOESNT. so i dont think thats the problem cuz most of the time the brakes work.... Is it air in the lines. or the master cylinder. I dont see any fluid coming out from the rear drums. so i dont think its a Wheel cylinder.... can somone throw me a bone? Thx.