To expand of what has been said here...Crankcase pressure IS BAD. Crankcase vacuum is good. A good evac design will pull a little vacuum which helps ring seal, staves-off flutter, and reduces oil in the chamber. But, if in doubt that you have siphoning from a mufflered exhaust then just run breathers vented to atmosphere, if you want to use a catch can then mount it where it might drain back into the engine. Any compromises, I'd put it in a place it's easy to drain. You could get all scientific and put a guage on the ports that you'd consider hooking up to and monitor it over the range of use.