The vacuum pumps that are sold by Blue Alpine have a solenoid in them to help prevent what is call back-streaming. At the end of the cycle the pump will usually turn on and off to maintain a vacuum. If the pump is not equipped with a solenoid, or if the solenoid is sticking, the pump can back-stream. This is where the vacuum of the chamber causes the pump to run in reverse and pull oil back up the intake of the pump. If you suspect the solenoid is sticking, you can plug your pump into a wall outlet and turn it on and off at 5 second intervals. This usually helps get the solenoid unstuck.