So it sounds like you must have the single for sure. Larry might have the piston vacuum pump with one line. Just pull off some of the vacuum hoses especially the one on top of the Injection pump going to the vacuum control module and see if you see any oil at all.
You can easily undo the brake vacuum line, and you will have to when you take your pump off anyway.
So is the diaphragm responsible for keeping oil out, or for creating vaccum in an oil free environment? Mine had a rip in it which resulted in a loss of vacuum, not oil in the pump. I belive the pump has a seal on it somewhere between the bearing that spins around and the flange it mounts to on the front of the engine.