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Master Cylinder Replacement
 
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Pelican Technical Article:

Master Cylinder Replacement

Tom Morr

Time:

3 hours3 hrs

Tab:

$288

Talent:

**

Tools:

12mm line wrench, 13mm wrench/socket, ratchet, extension

Applicable Models:

Mercedes-Benz R170 (1998-04)

Parts Required:

Replacement master cylinder

Hot Tip:

Use rags and brake cleaner to eradicate spills promptly

Performance Gain:

Better braking

Complementary Modification:

Replace brake booster

When brakes go bad, the master cylinder is often one of the last components to be fingered as the cause. Pads, rotors, and calipers are the usual suspects.

External master cylinder leaks spew clues, but internal leaks are harder to assess. The bad news either way: Brake fluid is highly corrosive. It makes paint bubble and isn't nice to skin or eyes. So, the worst part of master cylinder replacement is possibly dealing with the fluid and promptly dissipating/cleaning up drips. Covering the area under the master cylinder with rags or paper towels is a good idea.

The actual wrenchwork is straightforward. Two brake lines need to be disconnected from the master cylinder, and two nuts secure the reservoir/master cylinder assembly to the brake booster. Access isn't abundant, and the master cylinder needs to be removed level (straight ahead) to avoid damaging the thrust rod.

Internal leaks are often caused by the O-ring that seals the master cylinder's tail to the brake booster. When replacing the master cylinder, double-check to make sure than an old O-ring isn't still on the brake booster.

The most expedient way to do the job is to source a master cylinder/reservoir assembly (#005-430-77-01-MBZ for our project car, a 2000 SLK230 Kompressor). This is the approach shown here. Alternately, the existing reservoir can be screwed onto the new master cylinder, sealed with a fresh gasket.

Installation basically reverses the process. However, the brake system will need to be bled. Some Mercedes-Benz master cylinders include a bleed screw. This helps purge any air that might get trapped in the engine end of the master cylinder. Please refer to our other articles on brake bleeding and master-cylinder "bench bleeding" for further details.

Always use fresh DOT 4 fluid in your new SLK master cylinder. (Dispose of the old fluid properly.) Also, never pump the brakes when the master cylinder is dry. Damage can result.

The torque spec for the master cylinder mounting nuts is 20 Nm (14 ft-lb).

Begin by unplugging the fluid-level sensor lead from the master cylinder.
Figure 1

Begin by unplugging the fluid-level sensor lead from the master cylinder.

Use a turkey baster or a fluid pump to remove as much brake fluid from the reservoir as possible.
Figure 2

Use a turkey baster or a fluid pump to remove as much brake fluid from the reservoir as possible. Have rags and brake cleaner handy for clean-up duty. Wearing rubber gloves is definitely a good idea.

Two brake lines (arrows) must be detached from the master cylinder.
Figure 3

Two brake lines (arrows) must be detached from the master cylinder.

Stripped brake line nuts can add unnecessary time and turmoil to the job.
Figure 4

Stripped brake line nuts can add unnecessary time and turmoil to the job. To minimize the chance of this happening, use a 12mm line wrench on the fittings. Carpeting the area underneath the lines with rags or paper towels helps capture leaking brake fluid. Once the lines are free, cap them to keep dirt out.

Two 13mm nuts secure the master cylinder to the booster, underneath the reservoir.
Figure 5

Two 13mm nuts secure the master cylinder to the booster, underneath the reservoir. We couldn't get a bite on the upper one (arrow) with a 1/4-inch-drive socket, so we used a combo wrench to back off the nut a quarter turn at a time.

Pull the master cylinder straight forward until it's totally free of the booster.
Figure 6

Pull the master cylinder straight forward until it's totally free of the booster. Tilting the master can cause damage to the thrust rod. Some people err on the side of caution and remove brake lines from the ABS unit to assure a straight shot.

This photo shows the mounting studs' locations: roughly 7:00 and 1:00.
Figure 7

This photo shows the mounting studs' locations: roughly 7:00 and 1:00. The lower stud is difficult to see with the master cylinder and reservoir in place, but a socket will fit on its nut (red arrow). The brake module bracket (yellow arrow) also mounts on the booster's studs, in front of the master cylinder.

A screw (arrow) connects the reservoir to the master cylinder, for those who want to transfer and seal the existing reservoir to a new master cylinder.
Figure 8

A screw (arrow) connects the reservoir to the master cylinder, for those who want to transfer and seal the existing reservoir to a new master cylinder. Installation reverses the disassembly. Make sure that the thrust rod goes horizontally into the new master cylinder. Also, finger-thread the brake lines into the master cylinder to make sure that they aren't cross-threaded prior to tightening them. Thorough brake bleeding is required; please see our bleeding articles for details.

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