Air Compressor Pushes oil out intake

We are repairing our Ingersoll Rand EP75 compressor. We have made a new intake plate and replaced the brass pistons that hold up the plate. The machine turns on and builds pressure; but, it continues to build pressure after the air flow is cut off. We kill the machine and oil/coolant comes out the intake. 1. When the cover plate closes; what keeps the machine from drawing a huge vacuum preventing the plate from reopening? 2. When the power is shut off on a rotary compressor; how does it dump the built up pressure in the coolant tank? Thanks, Larry Clark MCH Manufacturing