All reciprocating pumps have two valves, an intake valve and a discharge valve. The joker valve is the discharge valve. The intake valve is the flapper valve at the bottom of the pump cylinder. The pump piston also has a seal around it. If that leaks by it also will not pump. If you look into the head while pumping you may get a hint of what has failed. If the intake flapper valve is failed you'll see water pushed back into the head on the down stroke. If the joker valve has failed you won't pull water out of the bowl, and the level will stay fairly constant. A failed piston seal would be indicated by it being real easy to stroke the pump, because it isn't rubbing against the cylinder. Hope that helps.