the suspension will try to self level for a while after you have left the vehicle, why i dont know, anyway this may be the reason it goes down, parked on uneven ground! or you may have a slight leak, either way when you start up the compressor fills a tank, if the suspension is low then it will try to pump up the R/R at the same time, rears first then fronts!
it will take a few minutes from empty but that should be it! if you have a full tank of air (11.5bar iirc) then it will be a lot quicker as you wont be working on air supplied from the compressor!
its sometimes good to start the R/R with a door open, this stops it from adjusting the hight, you can then keep an eye on the compressor and see how long it takes to fill ya air tank, when it stops close a door and up she should pop, and like V8 kenny said you may have to remind it what hight you wanted by pressing the hight switch again.
i got a complete assembly from island4x4 for £80, it comes built ready to install the new seal on new piston and crank assembly (i guessed this bearing would rear as well) and it even comes with new cylinder body too! i have heard since that the seals are very easy to damage when fitting and a friend of mine did indeed bugger his seal with out even knowing it!
AIR SUSPENSION PUMP COMPRESSOR REPAIR KIT ANR3731 K
£70 to your door!
i paid £80 a year ago and collected it myself!
P38 spares want nearly £90 !!