Air ride system not working properly

The system worked fine before it was parked. It has set for a few months. When you start the car the compressor wouldn't come on. The relay was bad so we replaced that. Now the compressor comes on but the rear of car still will not raise. During this you can hear a slight hissing sound near driver side rear tire. I can't tell if the sound is from air going in or air leaking out. All fuses and relays are good. Don't know if it could be the computer module or maybe level sensors.