Jaeger shutdown blowoff

We have a late 70's or early 80's Jaeger tow behind rotary screw compressor powered by a ford inline six cylinder diesel. Compressor is a model D, 150 cfm. I am currently trying to find a service manual for it. My question is, when we shut the engine down, there s a large sudden release of air immediately after, the air is usually loaded with oil vapor ( looks like steam) and then a squealing chattering slower release of air until all the gauges read 0 psi. Does this sound like pressure relief issues or loader valve problems? It otherwise compresses fine, with both enough volume and pressure, we just need to keep topping up the compressor oil due to the loss of oil at shutdowns. Thanks, Jack Jack Peacock, Picture Butte, Alberta, Canada