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Brake Master Cylinder Replacement
 
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Pelican Technical Article:

Brake Master Cylinder Replacement

Steve Vernon

Time:

3 hours3 hrs

Tab:

$40 to $350

Talent:

**

Tools:

11mm flared nut wrench, 13mm wrench, fluid pump, flathead screwdriver, power bleeder

Applicable Models:

Mercedes-Benz W123 (1977-85)

Parts Required:

Master cylinder, brake fluid

Hot Tip:

Use paper towels and plastic bags to contain any fluid leaks

Performance Gain:

Better braking

Complementary Modification:

Replace brake reservoir

Master cylinders will go bad over time especially if you do not regularly flush and bleed your brake fluid. Brake fluid is hydroscopic which means it easily absorbs water and this water can cause corrosion in the master cylinder, ruining the seals and leading to having to replace it. If you are continuously adding brake fluid to the system and cannot find where it is leaking there is a good chance that it is leaking from the rear of the master cylinder into the brake booster and collecting there. If your fluid reservoir is fine, you do not need to replace it as they are expensive. Just use care when removing it and always replace the two rubber grommets between the reservoir and master.

Replacing the master means you will be introducing air into the system; this also means you will need to completely bleed the system before driving the vehicle. Do NOT attempt to drive the vehicle before completely bleeding the system.

Begin by removing the brake fluid reservoir.
Figure 1

Begin by removing the brake fluid reservoir. The brake fluid reservoir (red arrow) sits on top of the master cylinder (yellow arrow). This area should be bone dry and if you find any brake fluid around here coming from the reservoir replace it while replacing the master.

You want to remove as much fluid from the reservoir as you can before removing it.
Figure 2

You want to remove as much fluid from the reservoir as you can before removing it. Use a suction pump or turkey baster to remove as much fluid as possible and while you may get a lot out you will not get it all so be prepared to catch and dispose of any extra fluid correctly. Brake fluid is deadly to the paint on a vehicle, so take every precaution not to get any on your paint and if you do flush the area with water and blot it up, do not wipe!

The red arrows indicate the fluid level sensors built into the reservoir and while they share the same fluid one checks the level for the front and the other for the rear brakes.
Figure 3

The red arrows indicate the fluid level sensors built into the reservoir and while they share the same fluid one checks the level for the front and the other for the rear brakes. Unplug the wire connections.

The reservoir sits in two rubber grommets in the master cylinder and is only held in place by friction.
Figure 4

The reservoir sits in two rubber grommets in the master cylinder and is only held in place by friction. Place a catch tray close so that you can put the reservoir in it as soon as you get it out from the master. Pull up while wiggling and the reservoir should separate. If it does not, you can gently insert a flathead screwdriver and gently pry up from between the reservoir and master cylinder (red arrow).

Your new master cylinder should come equipped with two new rubber grommets; if for some reason it does not, do not install it using the old grommets.
Figure 5

Your new master cylinder should come equipped with two new rubber grommets; if for some reason it does not, do not install it using the old grommets. Always replace the two rubber grommets that connect the master to the reservoir (red arrows).

You are going to disconnect the three hard brake lines connected to the master.
Figure 6

You are going to disconnect the three hard brake lines connected to the master. Always use the proper tool for this job--which is a flared nut wrench. The wrench is designed to grasp the fitting on five sides and make sure you do not strip it out. If you strip out the fittings you will have made an easy job much harder and a lot more expensive.

Use an 11mm flared nut wrench and loose and remove the three fittings and lines from the master (red arrows).
Figure 7

Use an 11mm flared nut wrench and loose and remove the three fittings and lines from the master (red arrows).

With the fittings loosened carefully pull the lines from the master (red arrow) using care not to bend them.
Figure 8

With the fittings loosened carefully pull the lines from the master (red arrow) using care not to bend them.

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

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

Carefully move the brake lines out of the way and pull the master from the booster.
Figure 10

Carefully move the brake lines out of the way and pull the master from the booster.

Your new master cylinder should come with a new rubber O-ring on the end; this O-ring makes the vacuum seal with the brake booster.
Figure 11

Your new master cylinder should come with a new rubber O-ring on the end; this O-ring makes the vacuum seal with the brake booster. If for some reason you are reinstalling the old master always install an O-ring (red arrow). You can see in this photo how our master was leaking from the corrosion on the bottom of it.

While it is very difficult to install the master incorrectly make sure that the center post from the brake pedal goes into the center of the master (red arrow).
Figure 12

While it is very difficult to install the master incorrectly make sure that the center post from the brake pedal goes into the center of the master (red arrow). You can also see in the photo the corrosion and damage from the leaking master. This will all need to be cleaned up before installing your new master cylinder to ensure a good vacuum seal with the booster. Installation is the reverse of removal.

You must completely bleed the braking system.
Figure 13

You must completely bleed the braking system. Do NOT attempt to drive the vehicle before completely bleeding the system. Please see our article on how to bleed your brakes including the use of a Motive Power Bleeder which makes it quick and simple.

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