the outlet check valve allows fluid to flow out of the pump, but not back in. When the diaphagm goes down, that valve should close, ensuring that fuel is sucked in via the inlet, not the outlet. Likely just a ball and seat. Rig up an adapter to the outlet of the pump to an air compressor fitting and attach to a leak down tester. That will tell you if and how much the check valve is leaking. Limit the pressure to what the pump is rated for, as too much pressure will rupture the diaphragm.
In theory, anything but the smallest of leaks in that check valve should prevent the pump from producing any pressure (any pressure produced on the up stroke would just cause that fluid to go back into the pump on the down stroke) and these don't typically fail intermittently unless a lot of debris is present. a bad exit CV would also make it impossble to create a vacuum to suck gas in on the inlet side.
Larry
