The check valve is there to prevent the water from being forced out
the city water hookup when the on board pump kicks on. It checks the
water going out, not in. When i took mine apart to check out what was
leaking i found that the PO didn't drain the line in the winter and it
froze. It cracked the fitting on the back of the hookup. When i put the
new hookup on it held with 30 lbs. of pressure behind it. If yours is
leaking, check for cracks. While your at it check and replace those
rubber gaskets around all 3 of the inlets.
  If you want to pull a fast one on your buddy. Have him push the check
valve in and tell him to look up into the outlet for obstructions. When
he's looking up there hit the fauet knob to run the pump. Your former
friend will certainly be impressed.