I have an eight foot lift so I don't want back flow. The check valve had eight feet of water pressing it closed - pressure that has to be overcome by the pump pushing air.

Solution 1) Set your bilge pump switch high enough so that you never suck the water below the pump cavity - no airlock.

Solution 2) Drill a very small hole in the top of the bilge pump cavity. Air will bleed out as bilge fills. Hole is too small to affect performance.

I use solution 1, recently saw a boat that used solution 2.