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Brake Master Cylinder and Reservoir Replacement
 
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Pelican Technical Article:

Brake Master Cylinder and Reservoir Replacement

Steve Vernon

Time:

3 hours3 hrs

Tab:

$65 to $200

Talent:

**

Tools:

13mm deep socket, 11mm flare nut wrenches, power bleeder, turkey'baster

Applicable Models:

Mercedes-Benz W124 (1986-95)

Parts Required:

New master cylinder and reservoir

Hot Tip:

Use paper towels and plastic bags to contain any fluid leaks

Performance Gain:

Better braking

Complementary Modification:

Replace brake booster

Without a doubt, your brakes are the most important systems on the car. The heart of the brake system is the master cylinder, which controls the hydraulic pressure of the entire system. Unfortunately, over many years, the master cylinder and reservoir have a tendency to wear out and leak. The leakage can occur internally or externally, resulting in a weakened braking system. If you have any problems with your brakes, and you think that it's related to the master cylinder or reservoir, you should probably replace it. Replacing the master cylinder on the Mercedes-Benz W124 is not difficult and it should take no more than three hours including bleeding the brakes.

The brake fluid reservoir can also get damaged over time and should be inspected with every brake pad change or once a year (whichever is less). This article will also show you how to replace the fluid reservoir.

When you are finished you will need to completely bleed the brakes. DO NOT drive the car without completely bleeding the brakes. See our tech article on how to bleed brakes for more info.

The heart of the brake system is the master cylinder (red arrow), which controls the hydraulic pressure of the entire system.
Figure 1

The heart of the brake system is the master cylinder (red arrow), which controls the hydraulic pressure of the entire system. Unfortunately, over many years, the master cylinder and reservoir (yellow arrow) have a tendency to wear out and leak. The leakage can occur internally or externally, resulting in a weakened braking system. 

Some reservoirs will have a line for the hydraulic clutch system (red arrow).
Figure 2

Some reservoirs will have a line for the hydraulic clutch system (red arrow). This vehicle has an automatic transmission so this line comes plugged from the factory. There is also a float or level sensor on the top of the reservoir (yellow arrow).

Before removing the reservoir or master cylinder you will want to try and get as much brake fluid as you can out of the fluid reservoir (red arrow).
Figure 3

Before removing the reservoir or master cylinder you will want to try and get as much brake fluid as you can out of the fluid reservoir (red arrow). Open the cap on the reservoir and remove the strainer. Using a fluid pump or turkey baster, get as much of the old fluid out as you can. This helps prevent excessive spilling of brake fluid inside the engine compartment. It's also helpful to stuff a bunch of old rags or paper towels under the master cylinder to absorb any spills. Brake fluid is deadly to the paint on your car so do not let it get in contact with the paint or anything on your person and then touch the paint.

Disconnect the level sensor (yellow arrow) and stuff a bunch of rags under the reservoir (red arrow) to catch the spillage when you pull the reservoir off the master.
Figure 4

Disconnect the level sensor (yellow arrow) and stuff a bunch of rags under the reservoir (red arrow) to catch the spillage when you pull the reservoir off the master.

The reservoir is only held in place by two rubber grommets between the reservoir (red arrow) and the master cylinder (yellow arrow).
Figure 5

The reservoir is only held in place by two rubber grommets between the reservoir (red arrow) and the master cylinder (yellow arrow). Wiggle and pull the reservoir from the master. If it is really stuck you can gently pry it up with a large flathead screwdriver between the reservoir and master. Watch for spillage. If you do get any on the paint dab it off. Do not wipe it.

This photo illustrates the two rubber grommets that hold the reservoir in the master (red arrows).
Figure 6

This photo illustrates the two rubber grommets that hold the reservoir in the master (red arrows). Always replace these grommets. When installing the reservoir put a little clean brake fluid on the reservoir connections to help slide them into the grommets.

There are two brake lines from the master cylinder (yellow arrows).
Figure 7

There are two brake lines from the master cylinder (yellow arrows). The brake lines are connected using 11mm nuts. There is a special wrench called a flared nut wrench that you should use to remove the lines (red arrow). The fittings on the brake lines are very soft and can round off easily. The flared nut wrench encloses the nut on several sides and makes the chance of stripping or rounding off the nut much less. These nuts are also fitted on the brake lines and cannot be replaced easily or separately.

This photo illustrates the flared end of the brake line (red arrow) that seals against the master cylinder.
Figure 8

This photo illustrates the flared end of the brake line (red arrow) that seals against the master cylinder. The nut is made of a soft metal (yellow arrow). Use a flared nut wrench when working with the fittings. Make sure the fitting is threading well by hand before using a wrench and never place any sort of Teflon tape or sealing compound on the nut.

Use a 13mm wrench and remove the two nuts holding the master cylinder to the brake booster (red arrows).
Figure 9

Use a 13mm wrench and remove the two nuts holding the master cylinder to the brake booster (red arrows).

Remove the master cylinder (red arrow) by pulling it out from the brake booster (yellow arrow).
Figure 10

Remove the master cylinder (red arrow) by pulling it out from the brake booster (yellow arrow). Use care not to damage any of the brake lines while removing the master.

The hollow end of the rod in the master cylinder (yellow arrow) must sit over the brake rod in the brake booster.
Figure 11

The hollow end of the rod in the master cylinder (yellow arrow) must sit over the brake rod in the brake booster. If you do not line this up correctly the brake rod will not depress the cylinder. If you are reinstalling the master always replace the seal on the back of it (red arrow).

Looking inside the brake booster you can see the brake rod (red arrow) that must sit inside the rod in the master cylinder.
Figure 12

Looking inside the brake booster you can see the brake rod (red arrow) that must sit inside the rod in the master cylinder. Installation is the reverse of removal and then bleed the brakes. DO NOT drive your car until you have completely bled your brakes!


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