A quick update on this; for those unlucky enough to have the C1A13-64 fault (Pressure does not decrease when venting the gallery") I still haven't found the solution but wanted to document what I've tried/learned so far to get rid of this problem... Certainly adding new O-rings to the three valve blocks has helped. There's a valve block at the front, one at the rear and a reservoir middle valve just in front of the air compressor (which in the US is in front of the rear driver side tire). The gallery is a term that describes air in the pipes of the system, so when the compressor isn't running and none of the valves are open it's the air in the pipes that go from (1) the middle valve to the compressor (a short pipe); (2) the air from the middle valve to the front valve block and (3) the air from the middle valve to the rear valve block. When the vehicle is lowered valves on the front and rear valve block are opened and the valve on the compressor is opened to vent air (the valve on the middle valve block is NOT opened, this is only open when the compressor is running and is only used to help fill the reservoir tank). I also observe that when the vehicle is raised the compressor comes on, the middle valve is opened (to start using the air in the reservoir tank) and all valves (front and rear) are open to pump the vehicle up; the front and rear valves are closed when desired height is established but the compressor continues to run until about 258 psi is reached in the gallery and reservoir at which time the middle valve is closed (so that reservoir has a store of compressed air at approx. 258psi) and then the exhaust valve on the compressor is opened and the gallery pressure drops to somewhere between 20 to 40psi. It's interesting! So in essence the reservoir tank is being used like a capacitor in an electrical circuit to store energy (in this case compressed air).
One common cause of the failure appears to be the exhaust solenoid valve installed on the compressor and given my concerns with the aftermarket compressor I went ahead and bought an OEM AMK compressor from the UK; having fitted this I'm still getting the C1A13-64 fault :-(.
My next suspicion was some sort of blockage in the exhaust venting system (creating back pressure), this is the bigger of the two pipes that connects to the desiccant tank on the compressor and the pipe system and exhaust that is attached thereafter. To negate this issue I tried disconnecting this pipe from the compressor (accepting that it will be noisy) but I still get the C1A13-64 fault! So this pipe:
At this point I took the LR4 to RoverLand in Detroit (they're really awesome) and they confirmed that the RLM (Ride Level Module) ECU is flashed for an AMK compressor and suggested I look for leaks in the pipes (which I've done and don't see any)!
I've watched numerous videos on YouTube and read tons of forums but still can't figure out what might cause this fault, perhaps the pressure sensor on the middle valve but watching it via an OBD2 tool (per previous comments on this thread) I'm not seeing it do weird things! I'm officially stumped (but learning a lot!)... If I make progress I'll report back, I'm grateful for other ideas!