I had issues with getting a lot of oil in the puke tank between runs, so I pressurized the crankcase with a smoke machine, it showed the rear main seal leaking and a small leak on the oil pan rail. And fixing those issues helped. But the bottom line was, how much engine blow by does the engine have? If you have more crankcase pressure from leaking rings, than the pump can handle, it will not pull any vacuum. I used a simple blowby tester that went into the valve cover , and it told the tale.