I have a 1999 DII, have had the air bags and valves replaced ($$$$), when the vehicle has been driven for over 3 hours, or has sat for a few days without being driver, or is parked on an incline with the engine in the up hill side, the rear air suspension deflates and the vehicle rests on the bump stops. When the truck is started and put in gear, eventually the bags re-inflate. Has anyone else had this problem? Any ideas as to what is causing it?

 

Leak. Mix up liquid dish soap and water in a spray bottle and spray things, look for small bubbles getting bigger. Just like testing an old inner tube. Fittings, lines, bags, etc.

 

Not necessarily, but that is the first thing to check. Mine used to do the same thing and new Arnott air springs didn't fix it, FYI.

 

Because it is happening after a 3 hour drive, you have an electrical issue and the only way to find it is to have a mechanic with a Test Book type scanner, that has Rover software read the SLS code and repair from there.
Let us know what they find before you get the repairs done, there are lots of used parts on the market that save you money.