A single 1.5" is only good for around 1400 gph (see RC drain calculator) and the Dart is probably pushing around 2000gph.

Valve back the output of the pump a bit (can restrict output no problems, but not the input side).

ok, so I raised the drain to just above the water and presto, drain valve 100% open, and I can get up to 40% on the return valve.

it is a little louder dropping into the sump but not bad

thanks to all

Looks like your running a Durso style drain. With the air bleed home on the 1st 90*. I have tried that way too, and didnt like it much. I found using a T, and having the air bleed on the other end of the T performs much better
If you look at the one in the pic here, you will see what I mean

http://www.dursostandpipes.com/

when a Durso air bleed gets pluged, it will surge in and out of a full siphon. Trick with them is to ballance the air bleed with the amount of water it should flow.

You don't need to run that much flow through your sump in any event. With your CL you've already got plenty of flow in your display. Using a lower flow return pump will save you a lot of electricity and your skimmer should work better.

Some like a lot, some like a little.

I'm on the side that there's nothing wrong with the return pump contributing to the overall turn over of the display (currently pushing about 1000gph through my sump).

Sounds like it is time to lose the Sedra driving the chiller and put the excess capacity from the Dart to good use.