The check valve to prevent water pressure from 'leaking' or back flowing into the water tank is usually right at the inlet to the water pump or actually built into the pump itself. If the pump keeps cycling and no other leaks can be found, it might mean the check valve is leaking. You then could either rebuilt the pump (if the valve is internal) or just add a new check valve in the line leading to the inlet of the pump.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI