I have a yr 2000 4.6 Vogue and its air suspension has always been temperamental. Recently it took the high setting of its own accord and then, a few hours later dropped to the bump-stops and stayed there. Without diagnostics equipment it is almost impossible to do any meaningful investigations. With a ‘box’ spares one could always do some swopping but that was not an option for me. I decided, therefore, to disconnect the air springs from the system completely and inflate each one manually. Firstly, I cut each air spring pipe from the compressor/valve assembly.
Using standard tyre valves,(stripped off the rubber moulding), to expose 4mm brass tubing, I then Super-glued (gell) one into each of the air spring pipes. A small chamfer on the brass tubing allowed it to enter each air pipe by about 10mm with minimal effort. After just 15 min I could manually inflate each spring. This should be done on level ground and some iterative pressure adjustments were necessary as there is a level of interaction as each level changes. There are manual conversion kits on the market but they cost several tens of pounds. Mine cost two!
Now for the rub! One of my air rear springs had a leak and the other decided to jam in mid position. Air pressure made no difference to height. Maybe these were the cause of the original problems. However, I now faced two choices: Replace the faulty springs or change to coils all around. Two air springs cost around 120 pounds and a coil kit around 350. I decided on the latter and bought a Brit Part kit from Brookwells.
Without help, I would allow yourself a day. You will need two axel stands and two trolley jacks and a fair amount of patience, but it’s worth it! The ride is firm but supple and a transformation from the choppy air spring original. Good luck.