For those contemplating issues with failing Hydropheunamatic compensating springs or are thinking of replacing with the spring I thought I would share my experience. I got mine from Niemoller in Germany to replace the spring.
https://www.niemoeller.de/en/w108/w108/B018/21/27 The car now rides correctly and adapts to various loads by maintaining rear end ride height. I've been told that the rest of the rear suspension needs to have good rubber and springs that aren't tired. If springs and/or spring pads are replaced be mindful to get the correct combination. Then when the hydro is installed the correct spacers are needed to get the rear camber correct. The BBB service manual details the procedure and specs.
It is an expensive part that probably needs to be exercised by driving the car regularly to reduce probability of failure. But hey the whole car does better with regular usage anyway.
Its a cool feature of these swing axle Mercedes Benz cars that enhances both the appearance and ride.