Its possible that the stem seal is leaking. Does it have a conical or flat seal? The old style conical seal stem is a bit fragile, and sometimes they get bent a bit making it impossible to get it to seal.
Its also possible that the seal between the upper and lower chamber halves is leaking. I find that the flimsy seal supplied with most seal kits is not adequate. A good replacement is the urethane o-ring that goes on the top of a paintball tank ASA valve. Make sure all the remnants of the old seal are gone also. The original seals were just heavy paper glued in place. Its important to get all the glue off the seal seat.
Another possibility, and one thats a real possibility, is the new cup seal. I've found a real inconsistancy in seal sizes with the new pump rods. I just did a 140 that had a new cup seal installed, it too was shooting in the low 500s. After trying a couple different new cups I got it to the upper 600s. The newer black hard rubber seals are quite often a bit undersized for the 101s also. You may have to locate one of the softer clear urethane cups that are a bit larger and replace the black seal.