The inlet connector on my pump had a brass insert, and it developed a micro crack and my system acted a lot like yours. I had to open two faucets to get water flow but as soon as I turn the faucets off that air leak would let the pump lose its prime AND the pump ran constantly. Even the tiniest leak on the inlet side will mess up the system, and I had to pressurize the inlet side to find that crack. I ended up replacing that inlet connector with a genuine plastic one, and never had a problem again. Craig.