Truck losing brake pressure at red lights

My 2004 Colorado, can stop on a dime with no problem. but if i am at a red light or in traffic, I will lose brake pressure. So i either have to push the pedal in further in intervals, or let off and re-apply. There is no visible leaks of fluid, the brakes have been bled, and the master cylinder has been checked. I keep rolling into traffic, and the only thing i hear from 2 shops is my foot most be falling off the pedal. Please help me.