I've been chasing a few issues with my air suspension recently and I'll tell you it can be a long patient game to narrow down the exact source of a leak. There are two options, you spend a ton of money just replacing random parts until the problem goes away, or you spend the time and narrow down the leak to one component. It is true that after 10 years a lot of the components will need to be replaced eventually but that's one big bullet to bite all at once.
While others have mentioned some of the workings of the suspension system, I've attached a overview pdf of the system. Disco3.co.uk can also be a really good resource for finding leaks. Search for "lowering overnight" or something similar to your symptoms.
Here's the basics to find a leak:
-Raise the suspension to off-road height.
-Pull Fuse 3 in the engine compartment, this will disable the suspension ECU which will prevent the computer from waking up and self-leveling over night.
-Measure all four corners of the truck with a measuring tape from the center of the wheel to the top of the fender liner.
-Depending on how slow your leak is it may take an hour or 8 or 10 to see a change. Measure all 4 corners again.
-Since the computer never self-leveled the car you can now determine these scenarios:
1: Front two corners lowered while rear stayed same or nearly same---Front Valve Block issue (Or 2 air struts are failing at the same time although unlikely)
2: Rear two corners lowered whole front stayed same or nearly same---Rear Valve Block issue (Or 2 air struts are failing at the same time although unlikely)
3: All 4 corners lowered---Front and Rear valve block issue (Or all 4 air struts are failing at the same time although unlikely)
4: One or multiple corners lowered significantly more than the others---One air strut is leaking at that corner, the bag or at the connector
If you need to change a valve block, consider buying a seal refresh kit ~$35 I think and replace the seals first. There are several other post here that detail that procedure. What happens often is that the desiccant dryer elements in the compressor break down to a fine dust over the years and gets distributed through the system to the different seals. That then breaks them down and causes leaks. If you replace the seals its a good idea to replace the dryer elements at the same time to prevent this happening again to your new seals. Those kits are on ebay, but make sure you know which brand of air compressor you have. Hitachi or AMK.
Once you determine which scenarios is most likely you can start tackling that one problem area. Sometimes, the issue will persist or you will find a new issue that was masked by that failing part. Again it's a slow game of wait and see and replace one component at a time until the system is stable. If you have this time you will save a significant amount of money over letting the mechanic replace parts willynilly.
Let us know what you find.
If you run a tire larger than this an EAS failure will leave you stranded even on a flat road.
