I had this problem for a while now, but was flushing brake fluid today and I though it will do some difference; but it did not.
Basically, when driving and ill brake in continuous motion all its fine. If I depress pedal, let go, depress let go and after third time my brakes will feel very hard almost like I have to push the pedal my self without any electronic or vacuum aid. all get back to the normal after few seconds of driving, but if repeat same pattern with brake pedal it will be "fading" again. I have noticed car behaves same after starting, depress pedal on the start and keep pressing its okay, but if I start, let the brake pedal go and move and want to brake, its "fading" again.- and again after few seconds of driving itll return to normal. this is a bit of a problem in traffic or when need to maneuver in tight parking spaces.
ps: no error codes