"I have checked the valves' positions, sealing washers etcetc until I am
blue in the face."
Simon,
Hope this is your BGO.
Have you tried blowing through the pump in both directions? You should
be able to easily blow through the body from the inlet/tank side, but
not the reverse.
I once did much the same dance with my pump as you are going through.
Only when I finally blew did I find that the valve preventing flow back
into the tank would not hold a sufficient seal. I'd inspected it long
and hard. If I recall correctly (this was 40 years ago), only upon
tearing the defective valve apart could I find the slight ding in a
hidden place on the sealing ring.
Regards,
Peter
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