This has been getting me in a state for a couple of week now

i have a 2wd saf cosworth and it has obtained a brake problem.
The car will brake hard like it should when you brake suddenly,BUT when you brake slow as at lights ect the car will brake as normal but the let go as if there is brake fade at the end of the brake pedal travel.
I have put new pads and disc on.
Any help will be gratefully received