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Replacing the Mercedes Master Cylinder and Brake Fluid Resevoir
 
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Pelican Technical Article:

Replacing the Mercedes Master Cylinder and Brake Fluid Resevoir

Steve Vernon

Time:

3 hours3 hrs

Tab:

$200

Talent:

**

Tools:

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

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. 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 W203 is not it is difficult, it is just a tight fit, but it should take no more than three hours including bleeding the brakes. The first step is to disconnect the battery. Next remove the 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.

Use a T20 Torx driver and remove the screw securing the reservoir from the master cylinder.

You'll want to mount the master cylinder in a vise, and then pry the fluid reservoir out of the rubber grommets on the top. It should pop out with just a little force.

Always install new gaskets when replacing the reservoir.

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.

When installing the master cylinder onto the brake booster take extreme care that the thrust rod

sits in the master cylinder, other wise you can damage it and/or your brakes will not work! Always replace the gasket between the cylinder and booster when ever you remove the cylinder.

Next, carefully position both of the brake lines into the new cylinder and thread in the brake line fittings. Be very careful when doing this as these are very easy to strip out. Go slowly and make sure they thread in easily by hand.

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 (yellow arrows) from the bottom of each cylinder.
Figure 8

Disconnect the brake pressure sensors (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.

Use a T20 Torx driver and remove the screw securing the reservoir from the master cylinder (yellow arrow).
Figure 11

Use a T20 Torx driver and remove the screw securing the reservoir from the master cylinder (yellow arrow). You'll want to mount the master cylinder in a vise, and then pry the fluid reservoir out of the rubber grommets on the top. It should pop out with just a little force.

Always install new gaskets when replacing the reservoir.
Figure 12

Always install new gaskets when replacing the reservoir.

Note: When you reinstall the master cylinder the lower nut is very difficult to access.
Figure 13

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.

Installation is the reverse of removal.
Figure 14

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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Comments and Suggestions:
Aldean Comments: Yall did not show how to unbolt the two nuts and linkage to brake pedal from underneath under the steering wheel
August 8, 2016
  Followup from the Pelican Staff: It isn't necessary to when removing the master cylinder. - Nick at Pelican Parts  
Frans Debono Comments: I need a new master cylinder MA0054309701for my Mercedes CDI 220 saloon year 2005. Can you please help me and give me an email address from where I can buy one in uk or Europe. Thanks for your cooperation.
Frans Debono
September 5, 2015
  Followup from the Pelican Staff: Give our parts specialists a call at 1-888-280-7799. They can figure out what part or repair kit you need.
- Nick at Pelican Parts
 
donf Comments: I have a 207 ml 320 cdi is the replacement similar to the pre 2007 model ?
July 22, 2015
  Followup from the Pelican Staff: I don't believe so. Procedure is a bit different. - Nick at Pelican Parts  
Simon Comments: Disregard the below - I meant to say, if switching out the brake fluid reservoir only, is it possible to do this without bleeding the brakes?
Ta
July 11, 2015
  Followup from the Pelican Staff: Reservoir, yes. - Nick at Pelican Parts  
Simon Comments: Following on from Du_man - can the master cylinder only be replaced without bleeding the brakes?

Thanks
July 11, 2015
  Followup from the Pelican Staff: Master cylinder, NO. - Nick at Pelican Parts  
Du_Man Comments: Can you expand upon Simon's question? I want to just replace the old crusty/cracking fluid reservoir. How do you access the bolt/screw on the left underside to free the reservoir? The body panel is so close to the reservoir to get a tool to reach underneath!
July 2, 2015
  Followup from the Pelican Staff: Fig 11 shows the fastener that has to be removed. Once removed, pull the reservoir up. - Nick at Pelican Parts  
Stefan Comments: Do you sell the crush washer and rubber grommet referred to in figure 6? Do you have part numbers for those?
June 23, 2015
  Followup from the Pelican Staff: I am sure we can get it for you.

I’m not the best with part numbers.

Give our parts specialists a call at 1-888-280-7799. They can figure out what part or repair kit you need.
- Nick at Pelican Parts
 
Simon Comments: Hi,

Is it possible to replace the brake fluid reservoir without removing the master cylinder/disconnecting the battery?
June 19, 2015
  Followup from the Pelican Staff: Just the reservoir, yes. - Nick at Pelican Parts  
Adrian Comments: Have you got a complete procedure for bleeding the brakes and replacing the brake fluid on this vehicle? I have a 2003 CLK 320 which I believe is the same.
August 26, 2014
  Followup from the Pelican Staff: We don't currently have that tech article, for the 208 chassis. This one is similar, but differences may exist between the two vehicles.- Nick at Pelican Parts  

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