Mercedes R129 SL500 SL320 500SL Convertible Top Hydraulic Cylinder O-rings Seals

The Mercedes R129 (SL-Class models like SL500, SL320, 500SL from 1989–2002) uses a hydraulic system with multiple cylinders to operate the convertible soft top, including main lift cylinders, bow extension cylinders, tonneau cover cylinders, front/rear lock/latch cylinders, etc. Leaks often occur from worn seals (especially the rod seals/U-cup seals), leading to slow operation, failure to raise/lower, or fluid loss.

Replacing the seals  is a popular DIY fix to avoid Mercedes' recommendation of full cylinder replacement (which can cost thousands). Many owners successfully do this with our upgraded aftermarket seal kit, though it's moderately advanced—expect several hours per cylinder, plus bleeding the system afterward. The main lift cylinders (the largest ones) are among the more involved.

Important Warnings

• This job involves hydraulic fluid (use Mercedes-Benz approved hydraulic fluid, often part #000 989 91 03 or equivalent like Pentosin CHF 11S—do not use generic ATF or brake fluid).
• Support the top mechanically during work to prevent it from falling.
• Work in a clean area; contamination can damage seals.
• If cylinders are heavily corroded, scored, or rods bent, rebuild may not last—consider a new/rebuilt unit.
• There are ~10–12 cylinders total; often people start with the leaking ones (commonly main lifts or bow extensions).

General Steps for Hydraulic Cylinder Seal Replacement

The process varies slightly by cylinder type (e.g., main lift vs. latch), but here's a typical overview based on common DIY guides and videos for R129 models:

1. Preparation
• Park on level ground, open the tonneau cover if possible, and manually raise/position the top to access the cylinders (use the manual override or prop it safely).
• Disconnect the battery (to avoid electrical issues with the top control).
• Have rags, a drain pan, and zip ties ready—fluid will spill.
2. Remove the Cylinder
• Locate the cylinder (main lift cylinders are in the rear storage area, behind trim panels; others near windshield, tonneau, or bow).
• Remove trim panels, welting, or covers as needed (e.g., door jamb rubber, top case plate).
• Disconnect hydraulic lines (use rags to catch fluid; note positions—lines often have clips).
• Remove mounting pins/clips (upper and lower; push clips open with a screwdriver, slide pins out).
• Carefully extract the cylinder (some have rubber boots secured by zip ties —cut or slide off).
3. Disassemble and Replace Seals
• Drain old fluid by cycling the rod.
• Remove the gland/cap (end with the rod seal)—often held by a clip, snap ring, or threaded (use a thin screwdriver or pick to remove retaining clips carefully).
• Pull out the old rod seal (U-cup), O-rings, wipers, piston seals, etc.
• Clean all parts thoroughly (no scratches on rod or bore!).
• Install new seals from a kit (lubricate with hydraulic fluid; The Ucap seals are replaced with standard seals to prolong life—U-cup lips.
• Reassemble (replace any O-rings on the gland/cap).
4. Reinstall
• Reverse removal steps.
• Reconnect lines securely (check for leaks).
• Reinstall trim and panels.
5. Bleed and Test
• Refill the reservoir (under rear seat or trunk area).
• Cycle the top multiple times (raise/lower fully) to bleed air—may need to do this with engine running or using the pump override.
• Top off fluid and check for leaks.
• Test operation fully.

Resources for Detailed Instructions

• YouTube Videos (highly recommended—visuals are crucial):
• "Rebuilding all Soft Top Hydraulics on Mercedes R129 SL500" — Covers accessing and rebuilding multiple cylinders.
• "SL R129 Top Hydraulics Repair (Main Lift Cylinders)" — Detailed removal and seal replacement on main lifts.
• Others for bow extension or latch cylinders (search "R129 [cylinder type] rebuild").
• PDF Guides:
• Top Hydraulics Inc. has excellent free removal PDFs (e.g., for main lift cylinders): Search their site for "R129 Main Lift Cylinder Removal" — detailed steps with photos.
• Forums:

• • Benzworld.org, MBWorld.org, PeachParts — Search "R129 hydraulic cylinder rebuild" for user threads with tips and photos.