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Replacing Your Mercedes Brake Pressure Sensors
 
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Pelican Technical Article:

Replacing Your Mercedes Brake Pressure Sensors

Steve Vernon

Time:

3 hours3 hrs

Tab:

$200

Talent:

**

Tools:

13mm socket, T20 Torx, flare nut wrenches, power bleeder large

Applicable Models:

Mercedes-Benz W203 (2001-07)

Parts Required:

New master cylinder

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. There are two brake pressure sensors on the master cylinder that can leak, wear out and send faulty signals.

Replacing the brake pressure sensors is not that difficult unfortunately you will need to remove the master cylinder to accomplish. The first step is to disconnect the battery. Next remove the fluid level sensor connector from the top of the reservoir.

You'll want to try and get as much brake fluid as you can out of the fluid reservoir on top of the master cylinder. 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.

Use a 12mm flared nut wrench and remove the two brake lines from the master cylinder. Make sure you use a flared nut wrench and make sure that it is seated correctly; you do not want to strip the nut. Also be prepared to catch the small amount of brake fluid that is going to come out.

You are going to have to remove the brake lines that connect to the ABS unit to get the master cylinder out. Disconnect the lines from the ABS unit, follow the lines where they pass through the sheet metal and remove them from the rubber grommets and set them aside.

There is a rocker plenum on the front of the master cylinder. Remove the crush washer that secures it to the rubber grommet. The crush washer will need to be replaced as it is a single use piece. Slide the rocker off the front of the cylinder and remove it from the car.

Disconnect the brake pressure sensors from the bottom of each cylinder.

Now locate the two 13mm nuts on either side of the master cylinder. These nuts hold the master cylinder in place. You'll probably need an extension with a universal joint in order to reach both nuts. Pull the master cylinder straight forward off the brake booster then tilt it slightly sideways and remove it from the car.Note: When you reinstall the master cylinder the lower nut is very difficult to access. Try placing a small amount of tape or a small piece of rubber glove between the nut and socket to help keep the nut in place. Just make sure the threads are clear.

The two sensors are located on the outlets for each brake line. It is best to put the master cylinder in a vice and use a large adjustable wrench to remove them.

Install the new sensors and reinstall the master cylinder. Now fill the master cylinder reservoir with new brake fluid and pump the pedal a few times to build up pressure. Do not pump the pedal without fluid in it. You could score and damage the inside of the cylinder bore, rendering it inoperable. You'll have to bleed all of the brake lines at each wheel to get all of the air out of the system. Please see our article on how to bleed your brakes.

DO NOT drive the car without completely bleeding the brakes

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

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. Our project car actually had a hole (red arrow) in the reservoir that allowed contaminants into the master cylinder ruining it. NEVER let your brake component get to this level of neglect.

Next remove the level sensor (yellow arrow) connector from the top of the reservoir.
Figure 2

Next remove the level sensor (yellow arrow) connector from the top of the reservoir.

You'll want to try and get as much brake fluid as you can out of the fluid reservoir on top of the master cylinder.
Figure 3

You'll want to try and get as much brake fluid as you can out of the fluid reservoir on top of the master cylinder. 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.

Use a 12mm flared nut wrench and remove the two brake lines (red arrows) from the master cylinder.
Figure 4

Use a 12mm flared nut wrench and remove the two brake lines (red arrows) from the master cylinder. Make sure you use a flared nut wrench and that it is seated correctly, you do not want to strip the nut. Also be prepared to catch the small amount of brake fluid that is going to come out.

You are going to have to remove the brake lines that connect to the ABS unit to get the master cylinder out.
Figure 5

You are going to have to remove the brake lines that connect to the ABS unit to get the master cylinder out. Disconnect the lines (yellow and red arrows) from the ABS unit, follow the lines where they pass through the sheet metal and remove them from the rubber grommets (green arrow) and set them aside.

There is a rocker plenum on the front of the master cylinder.
Figure 6

There is a rocker plenum on the front of the master cylinder. Remove the crush washer (yellow arrow) that secures it to the rubber grommet. The crush washer will need to be replaced as it is a single use piece.

Slide the rocker off the front of the cylinder and remove it from the car.
Figure 7

Slide the rocker off the front of the cylinder and remove it from the car. Yellow arrow shows where the master cylinder sits and the red arrow shows how it attaches to the car.

Disconnect the brake pressure sensors wiring (yellow arrows) from the bottom of each cylinder.
Figure 8

Disconnect the brake pressure sensors wiring (yellow arrows) from the bottom of each cylinder.

Now locate the two 13mm nuts on either side of the master cylinder (yellow arrow, one shown).
Figure 9

Now locate the two 13mm nuts on either side of the master cylinder (yellow arrow, one shown). These nuts hold the master cylinder in place. You'll probably need an extension with a universal joint in order to reach both nuts.

Pull the master cylinder straight forward off the brake booster, then tilt it slightly sideways and remove it from the car.
Figure 10

Pull the master cylinder straight forward off the brake booster, then tilt it slightly sideways and remove it from the car. Make sure it clears the studs (red arrows) before tilting it to remove from the car.

The two sensors are located on the outlets (red arrows) for each brake line.
Figure 11

The two sensors are located on the outlets (red arrows) for each brake line.

It is best to put the master cylinder in a vice and use a large adjustable wrench to remove them.
Figure 12

It is best to put the master cylinder in a vice and use a large adjustable wrench to remove them.

Installation is the reverse of removal.
Figure 13

When installing the master cylinder onto the brake booster take extreme care that the thrust rod sits into the master cylinder, other wise you can damage it and/or your brakes will not work! Installation is the reverse of removal. DO NOT drive your car until you have completely bled your brakes!

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