Make sure all your connections are airtight, especially INLETS; and optimum configuration allows for an airbleed. As air bubbles flow up, the inlet of the pump should be same level or lower than inlet vessel , lower than the pump outlet and the airbleed valve on the outlet higher. Turn on valves and bleed air out of the pump head before turning it on. If pump action doesnt happen after a few seconds, turn off pump, and rebleed air bubbles. Sometimes you have to move the hosing around to dislodge air bubbles. Once all air is purged liquid will flow.
Use a smaller 3/8 or 1/4" valve for airbleed at pump head outlet, teed off of outlet line, direct outlet of airbleed to a small pot. You want liquid to flow out before pump is turned on, close airbleed before starting pump. Look for nice solid liquid flow. Any liquid collected can be returned to batch.
Without an airbleed you have to move the airbubbles manually up to the head by raising sections of the inlet and outlet hose and moving them around a bit. Your inlet line should be short as possible. Solid lines definately require an airbleed arrangement. Hot wort , false bottoms screens and bazooka screens create a lot of air lock bubble issues. You should also have a throttle valve just at the pump head outlet to control flow. CHEERS , search byo pump arrangement for pic
