[The Radium King]

air in the crankcase isn't really metered air, so i wouldn't worry about trims or anything. i too have read that electric vac pumps don't like dirty air. one thing i have thought about, both for the aos and the brake booster, is the 987 tandem oil scavenge pump. there is a scavenge pump on each head, and the 987/986 pumps are interchangeable. the 987 pumps are 'tandem' because they also have a vacuum pump. the 987 uses them to provide vacuum for the brake booster (instead of taking it from the intake) but should work for the crankcase as well (they are just a mechanical impeller so should pass oil vapour, etc. ok). problem w most vac pumps is that they seem to all be designed to vent to atmosphere and don't have anything to connect a catch can too.

i think the bigger question is why you are chewing through aos's? i think the official party line is to confirm part # and always install the latest iteration of aos available (porsche has been frequently updating the part in an effort to address aos issues). so don't buy one that has been sitting on your local dealers parts shelf for 15 years and is not the latest version. and always go oem.

next, understand how the system works so as to troubleshoot. i got a great deal of info from a pair of threads on the rennlist 996 forum a few months ago. firstly, your intake system makes vacuum in two ways: at full throttle the air rushing through the intake pipe creates a venturi effect and a smaller pipe inserted perpendicularish into the intake pipe would see vacuum. this is the extra hoses you see on the intake on tip cars for example. 996's have these as well.

at idle the engine is trying to draw more air than it is being allowed (throttle closed) so vacuum is created; these are all the hoses on the intake that are after the throttle body. the aos connetion immediately after the throttle body tries to capitalise on both vacuum mechanisms (without dumping oil on the throttle plate).

so, crankcase wants vacuum, but not a whole lot of vacuum. in fact, the vacuum created at idle is too much. this is why the aos has a rubber diaphragm; as vacuum increases it sucks in the diaphragm and closes the connection between intake and crankcase. note that you want to be evacuating the crankcase at higher rpms and not at idle anyways; this is when the oil is getting churned up, turning to vapour, expanding, and creating back pressure on downward-moving pistons and blowing past rings or reducing power.

so why is the diaphragm failing for you more than others? something else going on w the system?