Repair & Maintenance · R129 · 1989–2001
Replacing the roof cylinders.
Twelve hydraulic cylinders raise, latch and stow the R129's soft top. Thirty years on, their seals give up one by one — here is how to renew them all in a weekend.
- Cylinders
- 12
- Fluid
- ZH-M · 1.25 L
- Difficulty
- Advanced DIY
- Time
- A weekend
How the system works
A pump under the trunk floor pushes about 1.25 liters of mineral hydraulic fluid through valve blocks to twelve cylinders in six pairs: front locks at the windshield header, rear locks beside the tonneau, a lock and a lift pair for the tonneau cover itself, the main lift cylinders that raise the top, and the bow extension cylinders that flip the rear-window bow as the roof folds.
Two design decisions make the system unusually friendly to repair. There is no residual pressure — switch the ignition off and every line depressurizes back into the reservoir. And it bleeds itself: air trapped during a repair simply vents into the reservoir as the top cycles. What ages is the seals; around thirty years old, they shrink and begin to weep, one cylinder at a time.
Reading the symptoms
Because each pair does exactly one job, the system tells you which cylinders are failing if you read the signs:
- Oil stains on the carpet behind the seats or in the soft top compartment — main lift or bow extension cylinders.
- A tonneau cover that sags, creeps back down or needs a helping hand — tonneau lift cylinders.
- Latches that no longer pull the soft top — or the hardtop — tight against its seals: front or rear lock cylinders losing force just past the pressure point.
- A pump that runs longer with every cycle, and a reservoir creeping below its minimum mark.
Parts and fluid
These are the factory part numbers for all twelve cylinders. Each hydraulic line is numbered to match its cylinder, so nothing can be reconnected wrongly. Two positions changed part during the run: the right bow extension cylinder (A 129 800 18 72 until 1991, A 124 800 02 72 after) and the tonneau lift on the earliest cars (A 129 800 03 72).
| Cylinder | Qty | Part number |
|---|---|---|
| Front lock — windshield header | 2 | A 129 800 16 72 |
| Rear lock | 2 | A 129 800 21 72 |
| Tonneau cover lock | 2 | A 129 800 00 72 |
| Tonneau cover lift | 2 | A 129 800 20 72 |
| Main lift | 2 | A 129 800 02 72 |
| Bow extension — left · right | 2 | A 129 800 17 72 · A 124 800 02 72 |
The system runs on mineral ZH-M hydraulic fluid to Mercedes-Benz sheet 343.0 — part A 000 989 91 03, sold aftermarket as Febi 02615. It holds 1.25 liters in total; buy two liters so there is enough to flush through while the system bleeds.
New cylinders have grown expensive and some numbers are no longer supplied. Specialists rebuild original cylinders with modern seal materials that outlast the factory parts, usually on exchange — resealing all twelve at once costs far less than chasing leaks one at a time.
Tools
- Torx and Allen keys — trim and cylinder mounts need nothing exotic.
- Small screwdrivers and needle-nose pliers for trim clips and the wire retaining clips.
- An 11 mm wrench for the reservoir's return line, if you flush the old fluid.
- Plastic bags, labels and a camera — photograph every connection before it comes apart.
- Rags and plastic sheeting: the work happens leaning over carpet, and the fluid stains.
The job
The order below does the easy cylinders first and saves the two awkward pairs for last. Experienced owners report six to eight hours to remove everything and about the same to refit — treat it as a weekend, not an afternoon.
Set the top up, cut the pressure
Park indoors with the soft top fully up and the hardtop off the car. Switch the ignition off: the R129 holds no residual pressure — with the key out, the valves open and every line depressurizes back into the reservoir, so nothing can move while you work.
Open up the compartment
Remove the two storage boxes behind the seats — four screws and two vacuum-line clips each — then the carpeted side panels of the soft top compartment. With the boxes out first, the panels come away without breaking their fragile mounting tabs.
Front lock cylinders
The two front lock cylinders live inside the latches at the windshield header, behind the header trim. Should the car need to be secured mid-job, the latches still close manually with the soft top tool through the access holes behind the sun visors.
Rear lock cylinders
The rear locks clamp the rear bow — and the hardtop — down beside the tonneau. Access differs by year: early cars reach them through openings in the trunk, later ones from behind the trunk's front trim panel. The cylinders sit inside the latches and can come out together with them.
Tonneau lock and lift cylinders
The tonneau cover carries its own pair of lock cylinders plus the pair of lift cylinders that raise the whole cover. All four count among the easy ones — clear access and short lines, good for building confidence.
Main lift cylinders
The pair that raises the top itself stands in the corners of the soft top compartment, working on the frame's main pivots. Access is tight and the lower mounts are fiddly — this pair and the next are why the job takes a weekend.
Bow extension cylinders
The bow extension cylinders flip the rear-window bow as the top folds, and they hide inside the folding frame. Move the top by hand into whatever position exposes them best and take your time — owners consistently rank these four rear cylinders as half the total effort.
Swap and reconnect
Work one cylinder at a time. Every line is numbered and every cylinder carries the matching number, so nothing can be crossed — photograph each connection anyway. The wire retaining clips slide off when you lift the center; ease them free without bending them, because each one is reused.
Refill the reservoir
The pump and reservoir sit under the plastic floor panel beneath the spare wheel. The min and max marks lie about 250 ml apart, so top up carefully toward max with fresh ZH-M and keep the bottle within reach for the next step.
Cycle, bleed, check
With the engine running to spare the battery, cycle the roll bar first, then the soft top, fully in both directions. The first cycles will be slow and noisy as trapped air vents back into the reservoir — the system bleeds itself. Six to eight full cycles usually do it; top up the reservoir between cycles, then wipe every joint and confirm it stays dry.
Before you start
- Only mineral ZH-M fluid to Mercedes-Benz sheet 343.0 — A 000 989 91 03 or Febi 02615. ATF or brake fluid destroys every seal in the system.
- Keep hands and tools out of the mechanism while the top is moving — the linkage carries real force and has no obstacle detection.
- Cap every open line and port immediately. A grain of dirt that reaches a valve block turns a seal job into a hydraulics rebuild.
- Protect the interior before you start: the fluid stains carpet and leather, and most of the work happens leaning over both.