The last few nights the rear/driver side of my 99 discovery II lowers. When I start the truck it raises to normal levels. I have researched this issue and found that it may be the air bag but the thing that throws me... It happens every night at exactly 9:15pm. Truck sits at work all day with no problems. get home at 5:30. Go out at 9:15 and watch the rear lower.


??? should I attempt to replace the air bag or is there something else I am missing?

 

Are you sure its not lowering 3:45 after you turn it off?

You can swap the bags and see if the drop follows the bag or stays on the left side.

 

Yeah... last two nights, right at 9:15PM

 

It's probably the SLS rear air bags suspension leaking or the compressor is defective or the NRV isn't working properly between the compressor and air suspension or it may be a case you need these guys

 

Did you spray the bags with soapy water to check for leaks?


Here is a good overview of the SLS system & troubleshooting:
Disco 2 Disco II Air suspension – the definitive guide! - Land Rover Zone