Sooner or Later, You're Going to Need Top Hydraulics

Raising & lowering that top in 20 sec is hard work 
You're doing something you've done many times before. You're raising or lowering the hydraulic soft top on your E320 Cabriolet when you notice a small puddle on your rear deck lid. (That's odd. I don't recall any rain.) You go to wipe up the small puddle, and that's when you realize it's not water. It's hydraulic fluid.

This is a classic failure. This is how the hydraulic system usually goes bad in an A124 Cabriolet (or R129 SL) after a couple of decades of use. The seals inside the Folding Top Rear Bow Lock Cylinder typically go first, but keep in mind there are 7 more hydraulic cylinders, so all of them will eventually need to be rebuilt.

Hydraulic fluid reservoir on
passenger side of trunk 
Last month, I experienced this classic failure. My Folding Top Rear Bow Lock Cylinder started leaking, so I got the answer to the question of whether my hydraulics had been restored under the previous owner. They hadn't. If I were pressed to guess though, I suppose I would have known that. In a car like mine that's pretty tidy, you can generally tell when something has been touched in the past since even the most fastidious repair will still leave telltale signs like marks from a wrench or screwdriver.

Mind you, you can't feel too bad about a 21-year old hydraulic system going bad. That's probably several hundred deployments. My understanding is, however, that even if you don't use the soft top these parts are prone to go bad. The seals simply break down over time. I've read some opinions that regularly refreshing the hydraulic fluid could extend the life of the seals, so if you haven't had issues yet, I would try this. If it doesn't work, all you've spent is about ten bucks on a liter of Mercedes ZHM hydraulic fluid. As this video shows, all you need is a turkey baster and a liter of hydraulic fluid from your local Mercedes dealer.
The Folding Top Rear Bow Lock Cylinder rests
underneath the bulbous convex cover. 
Sent Top Lock Cylinder + lock like this
to Top Hydraulics. No need to dismantle.

Made to order: the car was shipped to the US
the same month this part was produced.   
I immediately started searching for my options on the Web and quickly found the go-to specialist for Mercedes hydraulics: Top Hydraulics in Florence, Oregon. The experience was outstanding from the start. First, their website is excellent. It provided terrific directions for removing the parts, including dismantling the trunk lining. It's not a difficult process, but it's comforting for novices like me to see it all spelled out with photographs and step-by-step directions.

There are many outfits on the Web that offer hydraulic rebuilding services, including a local shop in nearby Danbury, CT, but Top Hydraulics was clearly the expert. Top Hydraulics doesn't just swap out the seals. It's a true upgrade. They replace the rod seal, piston seal, gland seal and port seals using superior materials. From their website: "Top Hydraulics, Inc. uses Viton and Teflon hybrid seals, as well as advanced PU seals which far exceed OEM's requirements for rigidity, but provide between twice and ten times the service life". Hard to argue with that.

Go to Mercedes forums and you'll see owner Klaus Witte is an active poster on the subject. Add to this that their pricing is competitive and turnaround is quick and it's a no-brainer. Plus they shared so much helpful information on the Web that I thought they had displayed a tremendous amount of goodwill towards the enthusiast community. (I always try to use businesses that share this kind of knowledge gratis. In my opinion, they deserve my business.)
Case Cover Locking Cylinder
with plastic cover removed

I decided to rebuild the two cylinders at the center of the car: the cylinder that locks the folding top down (the one that was leaking) and the cylinder that locks the tonneau cover into place. I did this because working on the leaking cylinder gave me easy access to the other cylinder. These also happen to be the easiest to access.

The task of removal and installation was pretty straightforward. A couple of insights to share should you attempt this job:
  1. Make sure the hydraulic fluid lines are firmly seated in the cylinder when you re-install. Otherwise, it'll make a mess.
  2. If the c-clip that holds the fluid line in place doesn't snap back into place with a reasonable amount of force, you'll know the line isn't seated deeply enough. 
  3. Case Cover Lock  
  4. When installing the Case Cover Locking Cylinder, there are 3 bolts that hold it into place—there's 1 on the bottom and 2 on the top. It's a tight fit in there, so install the lower bolt first, then install the upper 2 bolts.
All in all, this was not a difficult task for a novice like myself. Nor was it expensive. I paid $110 to have the two cylinders rebuilt, plus $10 to ship it back to me. I shipped them on a Monday and had them back in my hands the following Monday.

I fully expect a rebuild of the other 6 cylinders in the future, so stay tuned to this blog for updates. Regardless, Top Hydraulics gets my highest marks for an outstanding, highly professional service.