Originally Posted by Scrapper
If the above procedure fails to solve your problem you can go to a hardware plumbing section and buy an external check valve and install it on the output side of the water pump.
If you add an external check valve it should go on the inlet side of the water pump, not the outlet side. If the internal check valve in the water pump is leaking by, and you put the check valve on the outlet side of the water pump, pressure in the pump will just bleed off through the leaking internal check valve back into the fresh water tank and the pump will cycle on and off even though no water is being used.
If you put the external check valve on the inlet side of the water pump, the internal check valve will still leak by but it will only leak by back to the external check valve on the inlet, not back into the fresh water tank. This will prevent the water pump from cycling on and off when no water is being used.