Just purchased a 1978 Starcraft Starmaster 6 with what appears to be a brand new water pump. Filled the fresh water tank and hit the switch for long enough to get the bubbles out of the line - ran for a while then flow stopped. Bubbles were in the line so checked for leaks - found none. Hooked up to city water and water gushed out of the faucet and into the tank. Should the pump not stop this happening? Should there be a check valve somewhere in the system?