I'm installing dual Garmin G5's (AI + DG/HSI) in my Cherokee Six, which I plan to use regularly for IFR flight plans. I'm also installing an Avidyne IFD550 GPS/Nav/Com with AHRS. Since the G5's don't require vacuum, each have a 4 hour battery backup, and can serve as AI backups for each other... ...should I keep my OLD AI as a backup (along with the engine-driven vacuum pump)? Keeping the old AI would involve moving it to replace one of two old CDI's (the one without glideslope). Keeping the old AI seemed like a "no brainer," but more than one person has told me that vacuum pump failure is common. In fact, the vacuum pump failed just prior to the plane's pre-buy inspection. Do I really need the vacuum-driven AI as a "4th" backup to the second G5 and the IFD550? I'd love to have one less mechanical achilles heel to be concerned about.