Overflow box empties to fast.

[2bafish]

So I installed a overflow box with a bulkhead and and down facing 90 degree elbow. My overflow box empties fast, makes a gurgling sound, stops the input in my sump, water runs for a bit and the whole process begins again.
I only have a ball valve above my return pump but restricting the performance is a bad idea I heard because i butns out the pump faster. What can I do to fix this? Thank you