2008 Volvo with an Air leak that is driving me insane

Usually when the tank drains, the air bags will hold pressure for a while longer. I'm thinking you have at least one leak in the suspension system if the tank and bags drain together. Spray all fittings, all leveling valves and exhaust ports, spray the rubber and plastic parts of the air bags, sleeper air bags as well. There's also some pressure sensors behind the dash near the foot controls at the firewall pass through.

There are tons of other fittings and lines that are hidden, like under the floor near the seats and behind the dash.

 

The air bags would be my first suspect. The base will leak, can be hard to find. Spray up all the way around the bottom of each bag, where it wraps around the bottom base. They leak at the top of the base, especially the plastic base air bags. If that’s you’re problem, and they’re plastic base, try to replace with equivalent with a metal base. Next suspect in line would be leveling valve. Both can be intermittent and drive you crazy, thinking you’ve fixed the problem, only to return. Same with cab valves and bags. Might get lucky, during a good rain, or after hosing it down, sometimes a leak will make a high pitched noise when wet. Beats crawling around on the ground with a soapy spray bottle.