Vikas, there is no recommended replacement interval for the top hydraulic fluid. MB didn't go out 15+ years on the maintenance schedule. But that's about the age when original system fluid can start to reach its limits. Dirt, chemical oxidation/degradation, etc., and how often the top was cycled and the car kept all play a part. Most that complain of leaks were at or past that age on original fluids. Conversely, there are still a lot of people running original fluids that have so far had zero top problems.
Remember that they stopped making this model 12 years ago, and the earliest ones are now 25 years old.
It's not a perfectly sealed system, but has an easy access system reservoir in the trunk. Fresh fluid is clear. Once it starts turning darker, it's getting dirty and should be exchanged. Not an every two year thing like brakes, but perhaps every 5-7 years. Maybe even longer, depending on obsessiveness. There are probably also seal conditioners in the fluid that chemically degrade after that many years.
There's an outfit that offers reseal kits for when they do start leaking, which is cheaper than new cylinders. Once the cylinder starts leaking, then yes, the seal is done. But if you change the fluid every so often, you generally don't need to reach that point. All the MB people I've talked to about the seals claim it a red herring. It's not the seals, it's the fluid, and MB just didn't project new buyer service life that far out. For a brand new buyer in 1992 or 1995, this was a complete non-issue. Probably the same in 2002. Not so in 2014.
The reasons the windshield ones go is because the sun can beat on it all day, and it actually gets very hot in that area.
But the important thing is that changing out ancient fluid is a maintenance task, not a repair of something that randomly malfunctions. The top system itself just does not normally experience hydraulic or electronic control issues of any sort, even 25 years later. More people complain about the soft top plastic windows than they do about the top system itself.
FWIW, the automatic top system is separate from the auto roll bar system, which deploys in less than the blink of an eye if a possible roll over event is detected, or can be manually raised and lowered with a console switch. Truly an amazing setup, especially for 1989.
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The auto leveling (ADS) system on the R129s was pretty much confined to the V-12s. That is not a model I recommend, as there is absolutely no room to work on that engine in this car. Most R129s were V-8s, very trouble-free, and that is the preferred one to have, in my opinion. The handful of AMG R129s made are incredible, but most of them unfortunately didn't get across the pond.