Air is leaking from within the compressor pump--under the plastic shroud.  I can hear the leak.  Unit still builds pressure, but kicks on about every hour to hold a full tank.  Would replacing the Valve Plate Assembly and Piston Cup Assembly parts likely fix this?  Seems more often, units shouldn't be able to build pressure, rather than leaking when the compressor isn't running.  Any other parts I should order if this might be the fix?