I own a 95 TC.  I assume the systems are similar.  

If you hear the compressor running when you first start the vehicle,
that's normal.  The system vents after you turn off the car and close
the door.  I suppose it's a self-test procedure.  What isn't normal is
for the compressor to run often while driving.  

Make sure you turn the air suspension off before you work on your car!

Two easy ways to test for a leak:

1) Raise the car up and inspect the air springs for cracks.  Normally
the spring will crack near the bottom fold, but I suppose it could crack
anywhere.
2) Turn the air suspension off and drive the car for a while.  If the
rear sinks, you've got a leak.  

My original air springs lasted 9 years and 100,000 miles before they
developed leaks.  Your mileage may vary.  The symptoms I experienced was
that the compressor would run often while driving, and if I took a long
road trip the "check air suspension" lamp would come on about two hours
into the trip.  

I found the air bags easy to change.  You can probably do it one
afternoon if you are handy.  I bought some aftermarket springs first,
but they did not fit.  I ended up buying Ford springs and then the job
was easy.  The air fittings are a little difficult to disconnect, IMO,
but otherwise the job was easy.  If you have questions feel free to ask.  
I also replaced the compressor, dryer, and vent solenoids when I did
mine.   Total cost for parts was $600 including tax.