Sorry it has taken me so long to get back to this with some results. Right after I started checking into my vac pump problems I discovered I had a torn CV boot as well so the old girl has been sitting in the shop until I got un-lazy enough to fix everything. This time when I got the pump apart I discovered I didn't have a torn diaphragm but the nut that holds it onto the end of that little piston had stripped out and everything was loose. Perhaps I got a little over zealous with the ratchet when tightening it down. I checked the piston that actuates the diaphragm and it is a little loose in it's cylinder. Not horribly so but then it's not exactly tight either. Having never had one apart when it was new I have no idea how much play is acceptable. Just an estimate but I'd guess there's a millimeter or so of side play in it. To me it doesn't seem enough to be tearing up diaphragms, but sometimes it doesn't take much. For now I put a coating of grease on it to tighten it up a little and ran a die over the threads to clean them up and put a new nut on it. Band-aid measures I know, but we'll see what happens. If it keeps doing it I may bite the bullet and invest in a vane pump.
Just a note on what some guys were saying on pre-tensioning the diaphragm....... As I understand the way you were putting it you should run the piston to the end of it's stroke, whether all the way extended or all the way retracted wasn't specified (not sure that it would matter), so that the diaphragm is pulled or pushed to the extent of it's travel, and then tighten down the top cover. Is that correct? I've changed enough of these over the years that I'd gotten to the point where I just throw the new diaphragm in and had never really paid much attention to the tension on it. To be honest I've never had much trouble doing it that way. Up until recently I've never had much trouble with these diaphragms, maybe change one every 3 or 4 years, and I've had the old girl for about 17 years now. Anyway this time I followed the Bentley to the letter, and what they say to do is disconnect the piston from the driveshaft when you tighten down the top cover so there's no tension on it at all. I can't really see where it would make that much difference either way as there should be enough stretch in the diaphragm to handle the small amount of travel it goes through, but pre-tensioning seems to be the exact opposite of what the Bentley says, if I understand it right.