You aren't supposed to need to "add air" to anything in the LS400 except the tires, and there is no provision to externally fill the factory air suspension tank.
1. Black on Black 1999 LS400 PM-spec
-Maya-Megan-Magnaflow-LSportline-Xenondepot-PPE Engineering-Duce-Platinum VIP-Figs Engineering-R1Concepts-
2. 1989 Toyota Supra white package
Current Status: -500 Internal System Error-
3. Black 2004 IS300 -Manaray-HKS-Eibach-
4. Ebony Teal 1995 LS400 11-8-06
The first thing I would look at doing is getting soapy water and putting them all over the bags onthe side of the car the keeps falling/leaning and look for bubbles. If bubbles form you have a leak. If not then I would look at the sensors. Although the fact that the car keeps pumping air back into the mags makes me think the sensors are fine.