Vane type pumps do wear over time, ie., the housing itself can go out of round, which adds noise to the operating pump. The vanes can actually jump over the wear ramps inside the housing. This allows the compressed gas to leak back into the chamber behind, and represents inefficiency.
If it is the type of pump which I am picturing, the 'seal' as it were, is controlled by the gap between the rotor and the housing. I'm assuming the rotor is offset to one side, and barely clears the housing, with the inlet and outlet ports being on either side of this closest approach. Reboring the housing will smooth up the surface, however, it will also increase the gap between the rotor and the housing. This increases the amount of 'carry-through' between the inlet and exhaust, and so the pump is less efficient after such work is carried out. In some cases, the end-plates bolt pattern can be reworked to move the rotor closer to the housing again, or an eccentric sleeving operation could be carried out on the bearing bores.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)