How do i replace an air pump KU moldel in two compressors?

one pump has a case drain,this is a non pressure compensating pump.when the pressure builds to its setting,excess pressure is dumped out of the case drain.if the pump has no case drain,it is non-pressure compensating and will need some place in the system to relieve pressure or pump will fail first time it reaches set pressure.put a pump on that has the same number of hoses,capping the pressure compensating port will be a bad thing.