What are the common cause of rear air suspension?

Assuming you meant what is the common cause of rear air suspension LEAKS:

As the airbags age, they can develop leaks. That's probably the most common start to the problem. But it gets more complex.

Initially the automatic suspension leveling sensors and brain will have the air compressor pump more frequently in an attempt to keep the car where it should be. Eventually the compressor may run almost continuously, and it was never designed for that type duty, and will burn out.

So dependfing on how long and severe the failing suspension, it might be a case of replacing the airbag(s), or the vastly more difficult and expensive replacement of the air compressor as well. In my 1995 Town Car the compressor was at the front of the car on the driver's side, roughly underneath where the battery is located if I remember correctly. I think my town car was about 10 years old with 100,000 miles when the rear suspension failed. (By failing I mean it would no longer level the car...it was still at that time driveable.)

Rather than gamble and spend over $2000 on repairs (as my independent shop thought it was the computer that needed replacing, an analysis I disagreed with), I opted to order an air bag suspension replacement kit, consisting of coil springs and new rear shocks, from the web. I had them shipped to my mechanic's shop, they replaced the air suspension on a time-only charge, and I drove away happy.

I think the parts from the web and the labor totaled about $650, and had I been willing to take the time I probably could have swapped things on my own. From the instructions it wasn't that tough. By the time I drove a few miles home I was used to the coil spring suspension.

I was happy with this decision the rest of the time I owned the car. The only difficulty others have had is this may result in a permenant air suspension warning light on your dashboard. I didn't have that trouble because my coil spring supplier was aware of it and told me which wire at the computer board (located behind and below the glove box) to cut. (And before you ask: I know longer remember which wire, and no longer have the car to go look for it.)

Good luck,

Paul