Louis, if you left the brass retainer piece sticking out proud of the pump cup, then you've got excess 'dead space' in front of the piston cup which means the full gulp of air for each pump is not going into the valve chamber. Either pressing it all the way on or cutting it flush with the front of the cup will remedy this (also explains why your cup didn't clear the breathing hole with lever opened all the way: your overall piston head assembly is longer than normal). Obviously you'll need to re-adjust the rod length to compensate, so as to have the piston bottom out against the valve again.

Don R.