<<I have a friend who has replaced his mechanical pump on an IO-360 twice
due to low pressure.  It normally ran around 23 psi and was replaced because
it sporadically dropped to about 7 psi. We are considering the unique
apparent failure of a second pump after only 35 hours.
The fuel line was trouble shot for leaks in pressure (40 psi) and to a 20
psi vacuum and it is good to either tank.  Vent lines are also both clear.
I am looking for information about the nature of an engine side failure.
That is, does the cam? or pushrod?  device which drives the pump arm wear?
If so, is failure ascertained through measuring the throw of the plunger?
The second pump is also being sent back for bench testing.
Larry Henney>>

and:

<<Pulled injectors and tested flow and found air coming in from the boost
pump seal, it was a Dukes.  Replaced boost pump, air went away and the "bad"
engine pump was no longer bad.>>

and finally:

<<This just in.  We tore down the second mechanical pump and, lo and behold,
one of the two internal check valves doesn't check.  It allows fuel to
drizzle right back through it.>>

This is very interesting to me as I am having the same symptoms on my
Cessna.  Very occasionally it will quit for a moment with what appears to be
lack of fuel.  Almost all the components have been replaced recently,
although the "hiccup" didn't start happening until AFTER all the components
were replaced.  New electric pump, new mechanical pump, new gascolator
seals, new hoses.  Not likely to be air because it is a Cessna that keeps
pretty good pressure on the Dukes shaft seal.  Not likely to be water as the
plane is stored inside in California and it hasn't rained in months.  The
tanks were about half full both times this happened.  I removed all the fuel
hoses and inspected them for internal hanging chad.  I am leaning toward the
same cause Larry found - check valves that either stick open or stick
closed.  I'd like to hear the "final answer" on this one.

Gary Casey