>>SNIP>>For those of us with vacuum pumps (on my spamcan in this case), given
your experiences do you recommend changing the vac pump
"prophylactically", i.e. change it when it hits 500 hours or so (whether
working or not) as the recent SBs from the manufacturers recommend, or
do you recommend the "don't touch it if it hasn't broken yet"
philosophy? I have heard (not based on actual data) that they are likely
to either last 10 hours or 1000 hours.

Any insights to share?>>

The old style "wet" vacuum pump is more reliable than the "dry" pump. If I
were replacing a vacuum pump I would switch to a Garwin wet pump. My Bonanza
had a Garwin wet vac pump that lasted for well over 1500 hours. The down side
is the grime it creates on the belly of the aircraft.

I would recommend following the Service Bulletin advice about changing out
the dry pump at 500 hours -- but I wouldn't try any IMC for 25-50 hours just
to be certain that the pump and its components weren't contaminated with dirt
during the swap. Make sure you change the filters regularly, too. There is a
new vacuum pump out now that has a built in inlet filer to keep dirt out of
the guts of the pump.

If you do fly with a vacuum system you should consider a backup-- either a
backup pump or a second AI that is electric. I prefer the electric AI. Beech
and Cessna are have been  installing backup vacuum pumps on all new single
engine aircraft for a while now.

BTW-- there is a method by which you can lose one vacuum pump and lose the
other pump as well. If you have an electrically powered standby vacuum
pump--test it on the ground before engine start to verify it will run and is
capable of powering your AI. This also tests your shuttle valve that isolates
the two pumps from each other. If you have a Precise Flight system back up
vacuum arrangement then follow the manufacturers recommendation and test it
periodically. Once a month is not too often.

Another recommendation for vacuum pump fliers is to put a warning light in
your panel -- preferably just above your AI,  that illuminates if your vacuum
pump craters. A guage that is out of sight is not going to help-- unless you
tell your wife to keep her eyes on it and to scream loudly when it goes below
3.5".

Jeff