Why is there a check valve after the pump? Seams like a likely place for a clog to form.
The vent was my first thought also.
Lots of other places for problems.
Blockage between the pump and tank.
Bad valves in the pump.
My suggestion is to check the vent first. If that isn’t the problem, I’d look for a clog at the check valve. Next I’d look at the pump and suction line.
Be sure to let us know when you find the problem.
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Parks Masterson
Retired from Hopkins-Carter Marine Supply
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