What kind of check valve (3 basic types) do you have? I take it the you have a split system ac system being that it is an open drive. If so, is the compressor below the evaporator? Oil overcharging is worst than low oil level. You may have found your problem with spring loaded check valve chattering due to oil over charging an/or oil loggin. Condensing temperature is also critical for this type of accessory to work properly. Good luck