Christ Is Risen !

Well, there are 2 (check) valves in the pump.

The hand pump is connected only to the "suction space" of the pump .Pull on the hand plunger, fuel is sucked in the "suction space" via the inlet valve(#8). Push on the hand pump, the volume of fuel in the "suction space" (#7) is pushed through the outlet valve(#4) into the "pressure space"(#2).

Look at the highest point of the incoming fuel level (tank fuel level, supply line level) ,draw a line in your mind and determine the volume of the possible leak (the fuel needs to overcome the humble force of the inlet valve spring).