Today morning continue to debug the slow overflow issue.
Finally decide to add a air discharge point at the top U of the overflow.
Suspect the slow flow is due to air trap in the overflow highest point.
Get a one way air valve.
Drill a hole at the top U .
Cement it in place.
Setback the overflow pipe in the tank again.Top up water in the pipe
after shut off the ball valve at the discharge point.
Connect an short length of air tube to the one way valve.
Suck out the air slowly till water start to flow out of the one way air valve.
Close the one way valve.
Open the ball valve at the discharge point.
And you hear water start to flow out into the sump.Shut the top up point .
All this while the return pump is on.
After a while off the pump.
Water overflow into the sump till the water level in the main tank fall below the skimmer
inlet.
First test.
Water will not flood the sump.