Originally Posted by Banshee Ben
At what psi will the bags start to lower?
The pressure will depend on the load, it's not a fixed number. There are ride height control valves (usually three: one in front for both air bags which controls the front height, and two in the rear which control rear height and side-to-side lean.)
When the coach is lower than it should be, the valve opens to let compressed air into the air bag to increase the pressure and raise it up, and it closes when it reaches the right height. When the coach is higher than it should be, the valve opens to release air from the air bag, lowering the pressure and lowering the coach.
Why would one side sag. ?
If the rear air bag on that side has a leak, the pressure will drop, the coach will lower, and the ride height control valve will open to let in air and raise it back up. This will keep happening as long as there is sufficient air pressure in the air tanks.
But the air bag circuit is connected to a pressure protection valve (PPV) on the air tank. The PPV valve will automatically close when the tank pressure drops to about 60 PSI. This stops any air from going to the air bags, so that a leak can't drain all of the air and leaves some precious air pressure for the brakes.
So, that air bag is leaking, and the ride height control valve is keeping the coach level by drawing on air from the air tank. Eventually, the tank pressure gets below the safety threshold, the PPV valve closes, and the pressure in the bag can't be made up anymore. The pressure in the bag keeps dropping, and that side of the coach droops.
Slow air leaks are quite common. Two days is on the fast side for that to happen, but it's not unheard of. At that rate, it's not a safety issue, just an annoyance. The leak is slow enough that it will probably be hard to track down. Of course, if you wanted to find the leak, you would need to build up air pressure again before the attempt. Keep in mind that trying to find and fix the leak is potentially dangerous: while working on it, if you disturb the air line and inadvertently release the air pressure, the coach could suddenly drop and crush you.
You must have the frame supported with appropriately rated jack stands before working on an air suspension (or working on the hydraulic jacks.)