I already posted the following in the Benzworld forum ... but in the interest of those who are searching for tips on e320 Cabriolet top piston (cylinder) and bow piston removal, I'm re-posting it here.
My top failed late last summer as I attempted to put it down. The top had properly unlocked from the tonneau, but the back end of the roof wouldn't lift up more than a half-inch. Knowing this could be a big problem, I quit trying immediately. Next morning, I noticed hydraulic fluid in front of my passenger-side rear tire which confirmed I had a leak somewhere. I left the top up and in locked position all winter, then started gathering information on how to fix this myself.
I'm very grateful to Vexed, Kestas and KBelov and many others for the variety of posts (some going back several years) they have made in this forum. Reading their insights was an enormous help to me. For anyone else attempting this job, I have a few enhancements add to their previous comments. This post focuses mainly on the removal of the main top lift pistons (the long ones that lift and retract the top) and the shorter bow pistons (the ones that hike up the rear of the soft top, just before the tonneau cover for the storage compartment opens.)
Before I began, I ordered a couple of liters of the proper Febi hydraulic fluid from GermanAutoParts.com fluid since I knew I'd need it eventually. They charge only $10.96 per liter. Opening the trunk, I removed the fabric and metal cover on the right side that protects the top pump, then filled the reservoir with fluid (it was only about 1/3 full).
I then released the top catches on the windshield, started the car, put all four windows down, and hit the red top lift button. Alas, my hopes that Santa Claus had snuck in and quietly fixed the problem over the winter were dashed. As before, the rear of the top released from the catch but would not lift. And in checking the small plastic bowl I'd placed under the car in front of the rear tire, I saw fluid was again leaking. Trouble was, once I stopped trying to lift the top, the rear of the top reseated itself in the latch and locked ...which meant I couldn't manually lift it. So I tried the red button again, this time with a 1 x 4" board ready to slide in under the top base once it freed itself from the latch. It lifted as before, about an inch, and I quickly slid the board in to keep it from settling down and re-locking.
I shut off the car and lifted the rear of the top upward. I had a 4-foot bungie cord ready, and hooked that into the rear latch piece; I stretched the bungie over the roof and connected the other end to the front latch piece. Voila! Without using a bale of paper towels, I had a simple solution to keep the top propped open at both ends. Next, using the 10 mm wrench provided in the Mercedes tool kit, I manually unlocked the tonneau cover and raised it. Using another shorter bungie cord, I hooked the cord to the tonneau cover latch and then hooked the other end to the upraised trunk latch to keep both open.
It's an unfortunate part of the design, but you need to remove the back seat and interior panels simply to get at the one 40T Torx pin holding the main top lift piston in place. Removing the rear interior is fairly simple and straightforward. Begin by moving the front seats all the way forward, and tilt the seatbacks forward to give you work space.
Rear Seat: Press the red tabs below the front of the rear seat to release the seat, then lift up and out. You may have a bit of a problem negotiating the place where the seatbelt clasps pop up through the holes in the seat, but a few tugs and the seat bottom is out. Next, pull down the center arm rest and move the leather fabric in the notch where the armrest fits. You'll see two shiny hex head screws, 12 mm, up in the right and left corners. Use a 12 mm nut driver to remove these. Then, push or pull up the seat back. This was a bit more of a struggle than I'd anticipated ... but that seat has been there some 17 years. With some tugging left and right, it eventually comes up and out.
Next come the interior side panels. With the seat and seat-back out, remove the several black Phillips screws holding the side panels in position at the base and upper corner. With the widows down, open the doors and remove the shiny chrome door latch plate on the body (three very small Phillips screws that can strip easily). Then, move to the soft-top storage area and remove the Phillips screws that hold down the 3-piece black plastic trim that covers the tonneau latch mechanism (behind the roll-over headrest mechanism) and remove the left and right side portions. After that, you can lift up on the interior side panel and work it loose.
At this point you will want to use a 17 mm socket wrench to release the hex bolts anchoring the seat belts (front and rear) ... plus disconnect the plugged wires for the stereo speaker and the bow piston on the passenger side. Once the seat belts are unbolted, you can back-thread them through their respective holes in the side panel, and now the side panel can be removed and stored out of the way.
Next, there's a black plastic panel covering the side wall of the interior, plus a steel mount for the stereo speaker. The steel mount is held by only one or two screws and comes away easily. But the black plastic panel ... well, it has about 5 or 6 hex-heads holding it in (I simply used a nut driver) but worse, it's been sealed with black sticky gunk for sound deadening. My car seem to have a ton of this slathered on this part, far more than necessary. But that may be because I had the window regulator replaced at a dealer about 9 or 10 years ago, and perhaps they got carried away when they replaced the part. Just be sure to hand some old towels handy or a scraper to rub this nasty stuff off.
With the plastic interior panel removed, raise the rear quarter window. You can now easily see the lower portion of the main top bow piston sitting at an angle inside the hole. Just above, you will also see a black rubber grommet, about 1 1/2" in diameter, plugged into the metal of the side wall. This grommet is easily popped out with a screwdriver. And now -- at last -- you can see the object of all this interior removal work ... one of the two 40 T torx pins that hold each main top piston in place. As noted in other posts, these Torx pins were sealed with Loctite ... which leads me to a small digression and discussion of tools.
I'm no super mechanic, but I have a couple of old cars I keep running and generally own most tools one needs for auto work. Of course, every new challenge seems to require some dang tool I don't have, and that was the case here. I didn't own any metric Allen wrenches (hex keys) nor did I have a 40T Torx screw head. I bought both at a local hardware store.
Not having a proper driver, I inserted the 40T Torx screwhead into a 1/4" socket head and it fit nicely. Using an 3" extension and 1/4" ratchet driver, I put the ratchet-and-40T torx assembly through the hole and, of course, the 40T torx head promptly fell out ... and drifted down in the nether regions of the interior metal panel. Even using a magnetic retriever, I couldn't re-locate it. Argh. Back to the hardware store for another 40T ... their last one. This time, I put a bit of that sticky black gunk stuff (from the interior wall) inside the 1/4" socket, which served as a sort of glue to hold the 40T in place. It worked! As noted, the Torx pin was tight ... so, remembering Archimedes, I put a length of steel tubing over the ratchet drive arm to increase my leverage, and the Loctite seal was broken without stripping the Torx pin. After that, the torx pin came out easily.
However, rather than use my jury-rigged system, I urge you to purchase beforehand a proper set of metric Torx drivers that are designed to work with a standard ratchet wrench. Also -- and perhaps more important -- I urge you to buy a set of hex key (Allen wrench) drivers that work with a ratchet head, too. I used a standard "L" shaped 5 mm hex key to remove the pin that holds in the bow piston -- but it was very slow going, since I had to work blind, and by finger feel, withdraw and re-insert the hex key after each quarter turn in the tiny space inside the rear fender. Both a Torx and Allen key set can each be bought for about $16:
Okay, back to the piston removal saga.
Although I'd removed one of the anchor Torx pins for the main top lift piston, I decided next to shift gears and pull the bow piston. This was done working exclusively from the open tonneau compartment. The bow piston (which lifts up the rear of the top when activated) was the cause of my top headaches, and I could clearly see the Febi fluid that leaked from it.
I began removing the cir-clip that holds in the upper pin, using an awl to pop the clip off. BTW, it's a good idea to first put some cloth in the hole area below so the cir-clip doesn't disappear down the well. Once the cir-clip was off, the pin should slide out -- but with the rear of the roof raised and held up by my bungie cord, the pivot pin cannot be withdrawn -- its clearance is blocked by the black steel top mounting bracket. To solve this problem, I removed the bungie cord and let the top go partly down into the storage well; this action extracted bow pin piston shaft, moving it high enough so that it gained clearance from the mounting bracket, and then the pivoting pin was easily removed.
Next, I tackled the 5 mm hex pin holding the rest of the bow piston. As I mentioned above in the discussion of tools, this was probably the hardest task of all since I was using a standard L-shaped 5 mm hex key. I'm right-handed and, working on the passenger side bow pin, I really wished I had an extra elbow located in the middle of my forearm! In any case, after a few attempts and working blind, I found I could use my left hand feel my way down to get the hex key inserted in the pin, then give it a twist. As with the Torx pin, the first turn was the hardest and I feared I might strip the pin. But lucky for me, it came loose without problems. But this pin is tightly fitted for its full length (about an inch) ... and it took quite a while (perhaps 20 minutes) to painstakingly pull and replace the L-shaped hex key for a quarter-turn, using my left hand while bent over half way. Hence my advice about buying a proper hex-key ratchet system first. The job would be done in about 2 minutes with the right tool.
Once that pin is out, the only things holding the bow piston in place are the two fluid lines. These are easily removed -- use an awl or small screwdriver to slide the little rectangular clips back, then gently pull on the lines. You can then pull the bow piston up -- but be careful, the passenger side has an electronic plug attached and you want to be gentle and don't let it get snagged on anything when pulling it from the car body.
I really thought this was going to be a bear ... but it was surprisingly easy, and far easier than the bow piston. In fact, I had it out in less than 10 minutes after removing the bow piston.
As noted, I'd already removed the 40T torx pin from the interior side. There's a second torx pin located directly on the other side of the piston -- and the only way to reach it is just like the bow pin ... reaching down in the limited space from the tonneau compartment. Using my trusty 1/4" ratchet drive containing the "glued in" 40T torx head, I again used my fingers and worked blind to guide the head into the Torx pin socket. Using my left hand, I pressed down and -- bang -- it freed up without stripping. I then used the ratchet to loosen the pin further -- unlike the bow pin hex bolt, this freed up nicely. I didn't remove the torx pin entirely -- thinking of the task ahead of re-inserting it, I decided I spare myself that. So I just backed it off enough so that the main top piston was released.
You will need to work inside the car to disconnect the fluid line at the bottom of the main top piston. And you'll see the fluid line is doubled-up and clasped to the piston by a round black rubber holder. This just slides off. Then, return to the tonneau area to disconnect the other fluid line at the top of the piston. Last ... there's a very simply semi-circular brass clip at the top of the piston shaft. Just slide the clip art sideways ... and the top of the shaft is easily released. You can then extra the entire main lift piston. Total time for this was less than 10 minutes!
With those two pistons out, I quit for the day. Before tackling the driver side pistons, I decided to order the tools I've mentioned. Once they arrive, I wil pull the rest of the pistons, and then I'll do a follow-up post on reinstallation.