The rescue rover has been up and down suspension wise for as long as I have owned it. When first found, it was down to the stops, and barely had the oompf to get off the bumps...but when it did it defaulted to off-road height and wouldn't drop down until you get it over the magic 30 MPH mark, when it automagically lowers to normal. That was fixed by cleaning the selector switch and resoldering the switch pads and connector pins. As time and miles went on, I found the compressor struggled more and more until one fine day it gave uo due to a sticking exhaust valve, which I initially sorted with parts I swapped out of a FFRR compressor I had in my "should toss this out" recycle pile, but was glad I didn't. That got me another month or so, until...
I went to load up a new snowblower and decided that it would be awesome to go into the Gap tool and choose to go full access height there. The screen warned me that it could cause problems, that it might take longer than expected to fill the system and raise the vehicle but I'm as stubborn as I am impulsive and mash that button I did. Since I did that, the suspension has never actually recovered on it's own. Eventiually I gave up after trying to rebuld the compressor, before I fixed it I could get 52 psi out of it before it coded out, afterwards I could get 48 psi. Clearly not a useful trend. Anyways, I finally gave in and ordered a new unit from Amazon, found one on the resale side for $80, originally $199, described as "used-good. May not be in original packaging" Since I don't usua;ly keep the boxes, this doesn't matter to me, but there was something that did matter






The blue wire for the exhaust solenoid had come out of the connector. No connection there causes instant error and refusal to engage the compressor. If you were listening you might have heard the shout when I found this out!
I considered maybe pullig the pin and reconnecting the wire but I don't really have the tools to remove harness connector pins and figiured if I tried I'd just make a hash of it. Along this journey I had found that a FFRR Hitachi pump uses an opposite plug for the controls so you can't plug it together, and I had rewired the connector from an LR3 pump as needed....but since I had both genders of connector on hand I could build me a pigtail!

red wire goes to blue wire on pump
Having to make this pigtail meant having to find my soldering iron, solder and shrink tubing which I have cleverly hidden from myself in different locations. This expanded a half hour job at worst to an hour and a half, and unexpected profanities. My cuss jar overfloweth. Once done though, I have this!

pigtail and scotchlock as we call them here
So I didn't have to cut up my vehicle side harness.
So thus far, I have replaced the front springs, front and center valve blocks, and now done faffing about with wasted rebuild kits and just got a new compressor.

Along the way I did discover that if you have a Gap tool you can manually adjust the air springs. If you connect an outboard air compressor to the line going out of the pump and inmto the tank, you can raise the suspension to at least normal height and it will remain there. Do one end at a time and be patient-but by doing this you can avoid being stranded with emply air springs, and that is really the main argument for springs...which I have considered because a lot can go wrong. Be warned it doesn't like doing this and will bong incessantly.
Range Rover l322 pumps will work but you will have to change the 6 pin connector. Cut it off your LR3 pump. You might want to keep both plugs, just in case.
Future plans include adding a valve to make if possible to hook my portable Ryobi compressor I use for re-inflating my tires if I have aired my tires down. I found that a bad ABS sensor or wire will disable the EAS and drop it down to the bumps, and I'm sure other situations will do the same. Oh, and pressurinzing the system lets you check the galley sensing pressure too. It's another good reason to get a Gap ID tool.

Now I get to figure out why I'm mossing a signal from every other module to the all terrain module so I can have my Terrain Response back. I suspect the solution will involve dissamebly and a bit of De-oxit.

 

Since you have a GAP tool, you can force the compressor to operate and raise things up even if you have a hard fault that wants it on the stops. Then you just pull fuse 26 in the engine bay and move on til you can do an actual repair. If you want a way to fill the system, you can get a 6mm quick push connector then stuff a basketball inflator in there. The fitting will get stuck in the push connect for good and its a cheap easy way to keep an emergency method to inflation - and again, you can use the GAP tool to "force" bags to raise one by one and have your emergency compressor hooked up. GAP will not give errors, just force things to operate like valve blocks opening as needed.

You could use a T fitting as pictured below and keep it in place all the time - but this will not work. With the onboard compressor not working air will escape back out through it. Also the system operates are very high PSI, so I think leaving anything attached like this would be a bad idea. Plastic likes to break. Also why you generally want metal air fittings not plastic, they need to handle up to 250psi.

 

I did try using the test valve function to inflate the springs after the snowblower fiasco but apparently the compressor was not capable of raising the LR3 from the dead. It ran and ran until it stopped from overheating.
Looking for a suitable valve for the emercency supply, and how to attach it to the pipes, is definitely challenging as you pointed out from the pressures involved.
Part of my reason for writing it all out was to let folks know it is possible to do this. It's not common knowledge. I had to read the manual and men aren't allowed to read instructions.