Hearing a leak in the rear air suspension of a 93 MarkVIII Could a bad air compressor do this or could it be a leaky airbag or a faulty solenoid also Can they be manually filled to hold air?

You don't mention the mileage but if you're in the 80,000 range, more than likely it's the bag and they can't be manually filled. If left like that the compressor will probably be the next to die because it will be always trying to fill the bags and they are in the price range of $160.00 plus the labor. Could possibly be a solenoid but first put some soapy water all over the bags and see if you have a bubble thing going on then you will know.