Just purchased a 85 seville having a problem with rear air ride not leveling. how do i check &fix

1st I need to know what is wrong with it. How to fix is just to vague.

Check the Cadillac Forums website for details.

Ford Air Ride Rear Suspension Down Diagnosis & Fix ...

? 9:57
https://www.youtube.com/watch?v=wfh7n17hvN8
Feb 24, 2014 - Uploaded by FordTechMakuloco
Ford Air Ride Rear Suspension Down Diagnosis & Fix ... why theair ride compressor is not turning on and pumping up the air ridesuspension on your truck. ... Ford Expedition & Lincoln Navigator Air Ride C1725 Common Faults ... 2000 Ford Expedition - Replacing the rear shocks with load leveling shocks ...

2003 Lincoln Navigator air suspension fix(3) - YouTube

? 3:41
https://www.youtube.com/watch?v=MhWlcYycny4

Start the car listening for a hissing sound and looking for fluid leaking

Box is a heighth switch Eliminate current shocks and replace with old style air shocks and fill to desired heighth

Here's an idea to get you started.
Put the back end of the car up on ramps so you can crawl underneath it.
With the front tires blocked securely, have a helper get into the car and roll down the windows so they can hear you talk from under the car.
Have the parking brake set.
Take a bucket or container of soapy water and a brush, and / or a spray bottle of soapy water, and crawl under to car.
Have the helper start the car, and look and listen for leaks.
I have heard the air bags leak on air ride cars after a few years. The soapy water should show a leak when applied to the air bags and the air lines.

The ride level system uses an air compressor and either an air type of shocks or a type of air billow to raise and lower the vehicle, there is an adjustment to set the ride height and it is located under the vehicle and there is a control lever or rod that should be connected between the rear axle and a switch that is mounted on the body of the vehicle, and there will be an adjustment slot in the control lever between the switch and the body, and that is how you would adjust the ride level height, but be sure that the vehicle can not drop any further down if someone is under the vehicle trying to adjust the ride height.

However, you might find that the control lever to the ride level has fallen off from the switch on the body or it has come loose from the rear axle, and it is just hanging there not operating the switch on the body that controls the the ride level compressor.

Check for an air leak in the air suspension system, if there is an air leak the system will continually run until it burns out the ride level compressor.

Yes doe's your car, have the air ride suspension if so you need to replace the air pump or the air ride shocks on the rear or look for a valve under the rear bumper or in the trunk to inflate the shocks.. if nowhere to be found then take your car, in to a shop.
Good Luck!

Disconnect the - battery terminal for a minute and reconnect.
Good Luck