When I'm stopped at a light holding the brake pedal, it slowly inches it's way down and I find myself rolling forward...like it's losing it's prime. I have to take it almost all the way to the floor and then kinda pump it if I'm at a light too long.
Accelerate and go to the next light, pedal is fine at first...then inches down again, thus rolling forward a little.
With the engine off, does the pedal get rock hard and stay that way??
Sounds to me like you have a leak. It could be external, or could be the MC failing as fluid leaks past the internal seals. Does the MC look wet at the rear or fluid leaking on the booster?
I had a similar problem. Pull off the drums and see if they are pooling in the back. I have heard from a brake shop that this is a commen problem in 5.0 mustangs. Unless you have converted and I have wasted my time.