Rear suspension drooping. Air bag or???

[wsmeyer]

I've searched and read all the threads I could find but I'm still a little confused. My 2008 X5 with 3rd row seats drops down in the back after sitting for a while. Unlike most of the threads I've found, mine droops equally on both sides.

That leads me to three possibilities:

A. one bag is leaking and letting the air out of the whole system. But if that's the case why do some people only report one side drooping. Wouldn't both sides always deflate equally?

B. The check valve in the pump is bad and the air is leaking out there.

C. One of the air lines is leaking. I haven't seen a thread where this turned out to be the problem so maybe this is the least likely?

Hoping to get some guidance on figuring out what needs to be replaced.