no high pressure rv2 help!

guys" How important is the" evaporator pressure regulator"? Several people tell me that you cannot get that part anymore and any rebuilt compressors that you buy would would be without that component. Can a system operate without a EVR valve?????? This weekend I am changing the Expansion Valve per your advice . The book keeps referencing this EPR valve test that may be faulty that is not in my rebuilt compressor nor was it in the old original compressor on the car.I hate to keep opening the system up . Also loosing patience. Thanks Glen