Land Rover Repair: 2001 Disco2 SLS Sudden Drop, air spring, exhaust valve

Question
Hello, my 2001 Disco 2 had a leaky air spring, which I replaced and have enjoyed several months of driving on. Today I drove to work and everything was functioning fine. During lunch I left the parking lot and as soon as I cleared the driveway the car started dropping its rear, both sides equally until bottomed out.
Once home I climbed underneath and checked all lines. Everything seems fine, no cracks, breaks or disconnects. The SLS (lower left instrument panel) light is on steady. The pump spins up but there is no lift.
I am hoping it is something simple to fix but I am thinking the exhaust valve is stuck open. Is there a way to further diagnose these issues? Or would you have the answer as to what is wrong?

Answer
If the system is deflated the only way to test it will be to use a T4 or Autologic tester.  I can't imagine how one could determine if the exhaust valve was stuck from outside; that's really just a guess.  With the test system you can operate the valves and pump and see what happens.  However, I would have to say most people would just convert the rear suspension to coil springs with the kit from Atlantic British (about $400 I think) as tht puts a permanent end to these problems.