[strictlypersonl]

Usually, when there is a soft pedal when you press lightly, there's some contamination in the master cylinder. The dirt keeps the seal from keeping tight against the bore except when there's higher pressure.

Alternately, the free play in the rod between the booster and the master cylinder is inadequate, not allowing the return port to open consistantly when you release the pedal.

(As long as you're sure that you're not getting pad kickback from rotor runout or loose wheel bearings...)