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Brake Master Cylinder Replacement
 
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Pelican Technical Article:

Brake Master Cylinder Replacement

Time:

2 hours2 hrs

Tab:

$150 to $250

Talent:

**

Tools:

Socket set, flare nut wrenches, power bleeder

Applicable Models:

Mercedes-Benz W210 (1996-03)

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 one of 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 has 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, you should probably replace it.

Replacing the master cylinder on the W210 Mercedes-Benz is not difficult and should take no more than two hours. The first step is to disconnect the battery and the disconnect terminal in front of the passenger side rear seat. This is a safety pre-caution. You will be working close to the ABS pump and there is the possibility that an errant voltage spike could trigger the ABS unit and damage it.

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. Also disconnect the brake fluid level sensor connection on the top

It's helpful to remove the throttle control module from right in front of the master cylinder to give you more space when removing the master cylinder. Begin by prying off the ball socket on the end of the throttle cable with a screwdriver. Next, squeeze the two tabs on the throttle cable adjuster to release it from the mounting bracket. Once free, route the cable back through the mounting bracket and away from the module. Pull the brake booster vacuum line out of the rubber grommet on the module bracket. Now loosen and remove the three 10mm nuts holding the throttle control module in place. Lift the module up and turn it over. On the back side, you'll see the electrical connection. Squeeze the locking tabs to release the connection and set the control module aside.

Now locate the two hydraulic lines going to the master cylinder. Both lines use an 11mm fitting. You'll need to loosen and remove these fittings. The best way to do this is to use a flare-nut wrench. These special wrenches are designed to wrap around hydraulic fittings and prevent rounding off the nut. It's very easy to round off brake line fittings, especially if there is any corrosion present around the master cylinder. Break the fittings loose and un-thread those until you can pull each brake line out of the master cylinder. Make sure you have plenty of paper towels under each brake line to catch any fluid that spills out. Once free, place the ends of the brake lines inside some plastic bags to catch any brake fluid that spills out of the lines.

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. Once both nuts are removed, carefully move the brake lines out of the way and pull the master cylinder out of the brake booster. You may hear a slight hissing when you do this. Don't worry. What you're hearing is the brake booster de-pressurizing.

I prefer to remove the master cylinder with the fluid reservoir attached. It makes things a little easier and cleaner. Removing the fluid reservoir with the master cylinder attached can get brake fluid all over the place. 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.

Now take the new master cylinder and lightly lubricate the insides of the grommets on top with brake fluid. Remove the plastic protective plugs from the new master cylinder and pop the reservoir in until it seats fully. Now place the new master cylinder back on the brake booster and bolt in back into place. Torque the two 13mm nuts to 20Nm (14ft-lbs.)

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. Torque the fittings to 14Nm (10ft-lbs.). Now reinstall the throttle control module.

Now fill the master cylinder 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. (see Pelican Technical Article: Bleeding Brakes).

The first step to removing the master cylinder is to remove the throttle control module located in front of the master cylinder.
Figure 1

The first step to removing the master cylinder is to remove the throttle control module located in front of the master cylinder. This will allow you a bit more room to work with. Start by prying off the ball cup at the end of the cable (green arrow) and then squeeze the two tabs together at the retainer on the end of the cable (purple arrows). Now remove the cable from the module.

Remove the three 10mm nuts holding the throttle module to the panel (green arrows).
Figure 2

Remove the three 10mm nuts holding the throttle module to the panel (green arrows). Also carefully pull the brake booster hose out of the padded bracket on the end of the module (purple arrow).

Now carefully lift the module up to unplug the electrical connection underneath.
Figure 3

Now carefully lift the module up to unplug the electrical connection underneath. Squeeze the two tabs in to release the plug from the throttle module (green arrows).

Open the cap on the fluid reservoir, remove the strainer piece and using a vacuum pump or a turkey baster, try and get as much fluid as you can out of the fluid reservoir.
Figure 4

Open the cap on the fluid reservoir, remove the strainer piece and using a vacuum pump or a turkey baster, try and get as much fluid as you can out of the fluid reservoir. This will help reduce the amount of fluid that can overflow when removing the cylinder.

Disconnect the fluid level sensor on top of the fluid reservoir.
Figure 5

Disconnect the fluid level sensor on top of the fluid reservoir.

You'll now need to disconnect the fluid lines leading to the master cylinder.
Figure 6

You'll now need to disconnect the fluid lines leading to the master cylinder. Place some paper towels below the cylinder to catch any brake fluid. The connection on top (green arrow) is a 12mm fitting while the one on the side (purple arrow) is an 11mm.

Shown here is a flare-nut wrench being used to loosen the fittings on the master cylinder.
Figure 7

Shown here is a flare-nut wrench being used to loosen the fittings on the master cylinder. These special wrenches are designed to wrap around hydraulic fittings and prevent rounding off the nut. It's very easy to round off brake line fittings, especially if there is any corrosion present around the master cylinder.

It's a good idea to put the ends of the brake lines in some plastic bags to prevent fluid from dripping out (green arrows).
Figure 8

It's a good idea to put the ends of the brake lines in some plastic bags to prevent fluid from dripping out (green arrows). Also, use paper towels to soak up any fluid draining from the cylinder.

You'll want to use a 13mm socket on a universal joint to loosen and remove each nut holding the master cylinder to the brake booster.
Figure 9

You'll want to use a 13mm socket on a universal joint to loosen and remove each nut holding the master cylinder to the brake booster. The other nut is on the opposite side of the cylinder. Once both nuts are removed, pull the master cylinder away from the brake booster.

Once the master cylinder has been removed, you'll need to drain whatever fluid is remaining in the reservoir and then pry the reservoir out of the old cylinder.
Figure 10

Once the master cylinder has been removed, you'll need to drain whatever fluid is remaining in the reservoir and then pry the reservoir out of the old cylinder. Now transfer the reservoir to the new master cylinder.

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Comments and Suggestions:
vatch23 Comments: Will this procedure be the same on an mb608, or is the cylinder attached to the pedal in some other way?
August 29, 2014
  Followup from the Pelican Staff: This will not work for Benz trucks. I don't have any experience with that truck model to comment. This article applies to:

W210 E320 (1996-02)
W210 E300 (1996-97)
W210 E420 (1997)
W210 E430 (1998-99)
W210 E55 (1998-02 - Nick at Pelican Parts
 
steveo69 Comments: I have a 99 E300td. I have just replaced the master cylinder and bled the brakes, was all nice and firm now two days later the brakes are spongy again, on the 1st press of the brake pedal its spongy but if I press it again the brakes are firmer. Any ideas?
May 17, 2014
  Followup from the Pelican Staff: Sounds like there is air trapped in the system. I would start by bleeding the brakes again. Once led if the problem persists, check if you have a seized brake caliper or cracked brake hose. - Nick at Pelican Parts  
365merc Comments: I have a 1999 e300 turbo diesel I have heard that Mercedes will often use different numbers for different years/ models even though they are essentially the same part
February 27, 2014
  Followup from the Pelican Staff: You will have to cross-reference the part numbers. I do not think it will work, as they are different models. - Nick at Pelican Parts  
365merc Comments: I found a e320 2000 at the junkyard. I'm wondering if the master cylinder and break booster are the same. They look identical same connections same dimensions as far as I can tell. My car has 365k so I don't want to dump much in it. I also figured to replace both while I'm there considering the miles
February 25, 2014
  Followup from the Pelican Staff: You didn't mention what vehicle you have. If looking for donor parts, look up the part number for your vehicle and the donor vehicle. If the part numbers match, they will work. - Nick at Pelican Parts  

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