Worksite problem. Have two Atlas Copco GA 22 compressors with their own wet tank receivers piped into a common dry receiver. One of the compressors won't start and has a high temperature alarm. Has had oil added to it and loses it into the air system to fall out in pre-filters and after filters from air dryer system. Compressor can be depressurized and refilled to the wet tank from dry tank so the air is going reverse through compressor. Screw compressors are supposed to be positive displacement compressors and if this is the case, how can air travel from discharge back through suction? What needs to be replaced or looked at?