Reason I ask, is if that is the case then this is common occurrence with vacuum pumps, I would wager that every person once new to composites has experienced it at some point it isn't actually anything wrong, it's pure physics. The same physics that draw resin through a part in the resin infusion process actually. Without a valve, your vacuum pump achieves a higher pressure differential to the chamber you have pulled a vacuum on, and so you end up 'infusing' the oil out of your pump and into the chamber as the pressure tries to equalise.