Pump Check Valve cleaning & inspection
a very small piece of debris is all it takes to lose pressure.
Push the poppet all the way open with a screwdriver or a pen, and blow through the valve seat.
this is usually all it takes. If they bleed air AT ALL when you suck on them, REPLACE them.
Whilst yer there..
make sure there's no wear at the inside edges of the poppet-cage, where the poppet touches the cage.
The inlet valves wear sideways, as the sit sideways in the pump head.
If the valves are fine, but inlet valve cages are showing wear, switch 'em with the outlet valves..
it's like rotating your tires.
..And.. don't forget to check the o-rings under the check valves..
They flatten from wear, and allow pressure loss too !
Oh whatda heck.. ..while you're at it.. check for cracks in the head under those valve-seat o-rings too.
:{)