water pump help (fresh water)

i have a 2 way valve . normal flow & bypass . its in the normal flow position . if i close the valve (faucet type) the pump will dead head (stop pumping) as if it was pressured up . but that valve is close to water tank & is in supply line . have water but very little flow . checked faucet screens , no luck . just got me confused . any more ideas ? ....... thanks .. steve