moving beds sucking air

[Gallagher]

Has anyone here had trouble with the water pump sucking air from your moving bed filters? I can not turn the air on in the moving bed filters w/out sucking air.
The moving beds are DIY out of 55galand it doesn't matter how much the air line is open, it sucks air unless the air's off. The water pump is rated at 3400 gph and the feed line is 3" that reduces to 1.5 at the pump intake. I've tried both a vertical and horizontal pickup, horizontal works the best but I still get air. Any ideas?
Jim

[Birdman]

Can you put another barrel between the fluid bed filter and pump? This would get rid of the air before the pump.