Sizing a check valve

The starting means of my MS 5001 GT has the following connections from the tank: A check valve on the suction line of the fuel delivery pump and another one on the return line.

It has been observed that the suction check valve has gone bad. As a result, this does not allow fuel to be on the line during start up as efforts to use the hand pump returns fuel back to the tank. This has led to difficulty when starting the staring means. We have concluded that the check valve installed by the previous operators was incorrectly sized.

Now we are looking to get the specifications of check valve that will suit our application. Research has shown that a Cv value should be calculated from the flow rate, pressure drop across the valve and specific gravity. From this, you can size your valve.

But I have a problem attaining the pressure drop through the valve. Please any advice on what I should be do will be greatly appreciated.
"the suction check valve has gone bad"

Replace it; research not required.

And be sure the pump is operational and delivering pressure.