Load king dump bed

I recently purchased a tri axle dump truck with a load king dump bed. When I got home with the truck, I again tested the bed and lift gate, bed raises fine, tailgate unlocks, and lift gate raises, but when I lower the bed, the lift gate won't lower. I've operated plenty of lift gates, end dump, etc. but they have all had a separate switch for lift gate, this does not. The only way I can get the lift gate to close is to dump the air out of the truck. Can anyone help with this?