do you have the diffusers on the outlets or open bore plumbing?
I have a 45g tank with a sump capable of 175g tank filtering, I have a 950 GPH
pump on it, I also have a CPR 100 box I believe on there, I am unrestricted inlet and outlet with the same ball valve on there at full open and I dont suck dry at all.
I think your overflow might be too small to handle the pumps GPH
, This is my first sump system and i had it paired with the pump at the store I bought it from.
Here is a link which might help you a little more
its for the info not saying go buy their box, they just have great info on the overflows,pumps and sumps.
basically compare their boxes on the chart when you find your pumps GPH
and compare to other boxes even your own and go from there.
are you running a 2 outlet overflow box and still sucking dry or is there just 1 supply to the sump?