Penn-Plax Cascade 700 Losing Suction every 2 weeks

[dgcadm]

I've learned how to quickly prime and re-establish suction. All connections seem as tight as can be. With unit off, and lines closed, I remove the inlet line and then with line open, force feed with hose the line backwards and then immediately close the line. Then reconnect, open inlet and outlet valves and turn on the unit. Works quickly every time.

The problem is once every two weeks or so, the pump loses suction and pumps zero flow -- requiring priming the lines again. Pushing the prime button does not work, actually, never has for starting fresh.

I don't know how to tighten the connections any tighter. Perhaps need plumbers putty or tape to seal better??

 

[Delapool]

Nothing else changes in tank then? eg water change or power out or water level lower?

 

[dgcadm]

Nothing has changed. Power is continuous. Water level always maintained above horizontal outlet pipe. Threaded connections as tight as can make them.

Yet it must be bleeding in the tiniest amount of air, perhaps through the two threaded fittings on inlet side? So that over time, enough air seeps in to finally cause loss of suction. What else?

What would be the best plumbers sealant that would not harm/dissolve the plastic fittings? Or perhaps teflon tape for same fitings?