Try it without a checkvalve and see if you get better pressure.
If you put your airpump above the tank, it won't usually drain water out as the water won't travel upwards. To be honest, I've never used a check valve. lol.
20g platy, , 2 x 10g shrimp, 3 x 20g shrimp, 7.5g shrimp and 1 great dane/mastiff puppy.
Sump Pimp #2
My Tanks and my shrimps