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Remove the hose at the suction line,dip in a transparent bottle of fuel.Operate the hand pump.When you push,does it return fuel to the bottle?If yes,remove the lift pump and check the two disc valves ,clean the internals,renew the o-rings(air might be drawn on the lift pump suction/delivery stroke thru an old inlet nipple o-ring and along the threads.If the check valve is ok,there will be no visible fuel leakage,and still air can be drawn in the pump .Are the fuel hoses new?
You can use an appropriate vessel as a fuel tank to see if the problem goes away...and then search for the solution somewhere else(tank,metal fuel lines...).
Hmm,what else...check also if it`s not a hollow bolt used to hold the fuel return union(#306 and 312),as the pressure required for proper operation would never be maintained,even with by-pass valve ok.
If you`ve installed the later type fuel filter cannister with union on top lid
,check whether the hollow screw #261 has the 1mm.resctriction orifice-for the same reason.Not related directly to air presence,but worth checking.
See also whether the thin copper seal of the new hand priming pump does its job.On mine I could crearly see that it sucked air without leaking fuel and had to remove the lift pump to tighten it properly again.
I believe you got rid of the old metal pre-filter and used a plastic transparent one...
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