123 TD Wagon Rear Suspension Inspection Procedure ??

I own two wagons, an 82 and an 85. The 82 has rebuilt rear suspension in the form of new accumulators, shocks and leveling valve. The 85 has had the shocks replaced some time ago, but is now mushy and leaks down over a period of 12-24 hours. I'd like to perform a preventive maintenance inspection and performance test to determine the condition of the system and point to component failures. Where is a good place to start?