Fuel Pump check valve

[beckoning]

I rebuilt the fuel pump on my 1988 100hp 4cyl, oil injection. When I tried to blow through the small tube marked "in", it would click every time I blew into it. I probed into the opening and the probe stopped halfway to the inlet into the pump mechanism. Is there a check valve on the "in" tube on the fuel pump? Or is there crud lodged in the inlet tube going into the pump mechanism?