need some help, please.

ok, i think i have overlooked something and im questioning myself on this. im a hydraulic fan from way back and im not new to air, just not the most experienced.

i just built a taco on air and used extreme 250 1/2" valves. as soon as i aired up the system i realized i had a few air leaks. i left the truck sitting up and disconnected the air line at the tank--as soon as the tank was out of air the truck dropped. is this normal? ive never had this in the past. shouldnt the valves hold air on the bags no matter what? ive read about some people use check valves. should i have a check valve between the tank and the valves? have i just got a case of shitty valves or have i done something wrong? any help would be greatly appreciated.