Three failed tandem pumps! Ideas?

[NNH]

I'm at my wit's end after three failed tandem pumps in the last two years on my 2008 Cayman 2.7. For those of you fortunate enough not to know about this, the tandem pump sits on the front of the left hand camshaft bank, and is both an oil scavenge pump and a vacuum pump that produces the vacuum for the brake booster. On the 996/997 it hangs off the back of the right-hand head where it's prone to failing through conversion, so virtually all Google searches lead to that problem.

I've had three pumps fail now, leaving me with no brake boost, and needing powerfully-built PH director thighs to drive slowly home. The first two pumps failed on track days, in each case on the first lap out after a break. The third pump failed after about 30 minutes cruising on the Autobahn at 100-130mph. In each case, the vacuum section of the pump had failed after filling with oil which hydrolocked the pump and sheared the input shaft.

I put the first failure down to bad luck. The second was diagnosed as potentially due to an elderly airil separator, which was also replaced. The most recent one has me mystified, as although the engine was running at around 4000-5500 rpm, it was hardly overstressed and at -2 Celsius outside I don't think there can have been any overheating.

GT One in Surrey have been in touch with Porsche GB, who are also mystified but did comp a new pump.

HELP! I can't fit a new pump every year, and I don't like feeling nervous about the brakes at speed. Has anyone come across this problem before?

TLDR: Brake vacuum pump keeps failing at high revs/load, which is just when I need it most.