Rear airbag(s) replacement?

I have recently had problems with the suspension on my 1996 P38. When the car has been left for more than 2 days, the rear of the car sits down on its bumpstops, front still ok.
When it is started, it takes a couple of minutes to build up the air so it rides level again. The EAS system works as it should.
However, if I leave the car for any longer, say a week, the rear still sits down on bumpstops but it then has difficulty building up air when running. The EAS lights just flash, the ride height indicator (high level light) on the dash is lit and it doesn't change even after several minutes of driving. I have driven 15 miles in this state recently. If I then leave the car for a period of a couple of hours and restart, everything is normal again - almost as if there is air building up when driving but not registering until it is reset from restarting engine.
I want to replace the faulty components but a little unsure what to replace and how to do it. How many rear air bags are there and does it sound like the problem is with it/them?
Is there a sensor which operates when the air is too low and also when it reaches level or pressure?
Any suggestions??

Replace all 4, ie the 2 front and the 2 rear as on a 96 car they will all be perished if original. I do a guide with photo's, if you want a copy, PM me with an email address.

Besides the airbag leaks sounds like you have leak on the pressure storage side and a weak compressor. After running when parking up there should be stored pressure retained in the storage system, the vehicle should lift almost instantly when engine is started. This leak needs to be found ( leak in line from compressor to air tank and back to valve block) First of all change bags as Data says and see what you get.

You can check the entire system (air lines and airbags) by spraying soap water and watching for leaks. This will help you identify the problematic areas.