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This is already my 3rd vac pump failure in just over a year. First time was a little over a year ago at night enroute to Florida. Ended up continuing 3.5 hours in night VMC with no AI/AP and over 6 hours back home in day VMC. Second time was about a month ago on the ground after landing VMC. Third time was last night on the ground just after start up (less than 20hours/30days since the last one was replaced). What the heck!? We flew out to Pittsburgh to see a friend and were heading back same evening. We were rushing all day to get back to the airport before 8 when the line service closes to return the car. We ended up running 15 minutes late and were afraid we'd get stuck the night. But it just so happened that the line guy was waiting for a late jet arrival soon so we still made it. He let us drive up to the plane and everything was going great. I fired up and was fiddling with the fuel totalizer and within about two minutes of start up, I notice the 2 vac failure lights and AI keel over. Standby vac that works from differential pressure worked (but this only works effectively in the descent when you throttle back and not useful for takeoff or enroute). It was totally clear VMC. But coming back over some mountains to NYC with an already failed system (plus tiredness and other factors) was not a good idea so we had to stay the night and go back in the morning. The morning departure was rushed because deteriorating ceilings and snow were coming soon. Are these vac pumps really so defective or is it possible that something is wrong with my plane that breaks them? As far as I can tell, since they suck air and since they are driven by a gear, it's really just the pump failing and everything else is ok. The standby vac that works off differential pressure worked, so that pretty well demonstrates that it's not the result of leaks or problems in the lines. AI worked with the standby so not a problem in the AI either. Anything worth checking or diagnosing before replacing the vac pump for a third time? I don't want an electric vac pump because I have an electric HSI. So the only vac driven instrument I have is the AI. I figure it's better redundancy and cheaper to just have a backup AI. Now as to backup electric AIs. Can you use any of them or are there different levels of certification? So far my biggest problem with vac failures (and as probability would have it) was not that they dangerously happened in IMC but that they would keep me on the ground waiting for good day VMC instead of departing when desired. Would having a backup electric AI legally allow me to depart night VMC or any kind of IFR? Or are some of them advisory only, not for exclusive use, or not IFR? How does the type certificate impact departing under the electric backup with the vac system inop?