Volvo 780 front air bags deflating

Its your leveling valves so dont check the lines or the tank the valves are one way they have no way of releasing pressure back to the tank or lines there is a valve on the bottom that releases pressure these valves go bad after a while I my self would worry about it it wont put you oos but if you want to replace them the cost around $150 a piece and not to hard to replace

 

but before you replace, measure ride hight and when new valves be installed reajust ride hight

 

the is a pressure relive valve bolted to the frame under the fifth wheel (300 bucks and 1.5 of labor) check that