Not knowing this pump, but having heaps of experience on old car ones, check
the flow with the output line disconnected to see if it does flow correctly
each stroke.

Valves and passages can gum up with deposits and restrict flow, alloy
housings can corrode causing similar results. My old 1964 Series 2A
Landrover hand pump will not prime the lines if the carby bowl is full as
the needle valve woon't open and the output line becomes pressurised.
Relieving the pressure by loosening the inline fuel filter lets the air out
and the line (and pump) primes happily then.

The pump on another old ex-army truck had a mesh screen that was full of
junk that had to be scraped out with tools. This truck had only 30,000 miles
on the clock, but the old fuel had still caused it to corrode and gunk up.

How much fuel, and how many years of crap has been through your tank and
lines? How hard do you shake up your tanks when flying a hammerhead? If
there is a flaky rust spot on the side of the tank, it's sure to come loose
when you least expect it, probably in straight and level flight.

If the pump can be taken apart, I'd tend to have a look, even if only to
feel comfortable with the operation that your life may depend on.

Hope this helps,
Peter