I think your not re-charging the master at the begining of its stroke. it sounds like you have flow, but the master is not returning far enough to take in more fluid. check your brake light switch to see if its holding the pedal down some. it maybe just enough to cause this head ache. secondly use the power booster. when you cycle the system the accumulator lets you get between 5-8 pumps under power with the engine off.