this man speaks the truth i had a hatch that did this and i ended up replacing the whole f'n system to fix it, I tried pads rotors, calipers, prop valve, booster master, and then it worked great.
start with the master cylinder, the pushrod from the pedel/booster may need adjustment i.e. go to honda because brakes-r-us places dont have the required tools to fix or check the clearence
g/l