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Pelican Technical Article:

Bleeding Mercedes Benz Brakes

Steve Vernon


2 hours2 hrs





Applicable Models:

Mercedes-Benz W123 (1977-85)
Mercedes-Benz W124 (1986-95)
Mercedes-Benz W126 (1981-91)
Mercedes-Benz W201 (1984-93)

Parts Required:

3+ quarts of brake fluid

Hot Tip:

Use different-colored brake fluid so you know when your system is flushed

Performance Gain:

Quicker, firmer stopping

Complementary Modification:

Rebuild brake calipers

Bleeding brakes is not one of my personal favorite jobs. There seems to be a bit of black magic involved with the bleeding process. Sometimes it will work perfectly, and then other times it seems like you end up with a lot of air in your system. The best strategy to follow when bleeding your brakes is to repeat the procedure several times in order to make sure that you have removed all the trapped air from the system.

The basic system can be bled using traditional method. There are currently two popular methods of bleeding the brake system: pressure bleeding and vacuum bleeding. Pressure bleeding uses a reservoir of brake fluid that has a positive air pressure force placed on the opposite side of the fluid, which forces it into the brake system. Vacuum bleeding is where you fill the reservoir and then apply a vacuum at the bleeder nipple to pull fluid through the system.

The method that I've come up with combines the first method described above and yet another third finishing method. I advocate bleeding the system with the pressure bleeder and then using a family member to depress the brake pedal to free up any trapped air in the system. If your esteemed assistant really owes you a favor, you will be the one depressing on the pedal, and they can spill brake fluid all over themselves.

The first step in bleeding your brakes is to jack up the car and remove all four wheels. The next step is to fill the system with brake fluid. I recommend using a quality brake fluid, such as ATE SuperGold. One of my favorite tools for pressure bleeding is the Motive Products Bleeder.

The system has a hand pump that you can use to pressurize the brake fluid to just about any pressure. A small gauge on the front of the brake fluid reservoir indicates the pressure of the brake fluid inside. The very large reservoir can hold about two quarts of brake fluid--more than enough for most brake flushing and bleeding jobs. The bleeder kit is a very useful and cost-effective tool to have in your collection.

The system bleeds by pressurizing a bottle filled with brake fluid from air from an internal hand pump. The procedure is to add fluid, attach the bleeder to the top of the reservoir cap, and pump up the bleeder bottle to about 5-10 psi using the hand pump. This will pressurize the system. Check to make sure that there are no leaks around the bleeder or where it attaches to the top of the master cylinder reservoir.

Now start bleeding the system. Start with the right rear caliper, the one that's located furthest away from the master cylinder. Bleed the right rear caliper by attaching a hose to the bleed nipple, placing it in a jar, and then opening the valve by turning the bleeder nipple counter-clockwise with an 11mm wrench. Let the fluid flow out until there are no more bubbles. If you don't have a pressure bleeder system, you need to find someone to press on the pedal repeatedly to force fluid through the system.

Another solution is to get a check valve and place it on the nipple while you depress the pedal. This will work for getting fluid into the system but you will still need a second person for the final step to make sure you have bled the system completely.

When no more air bubbles come out, move to the next caliper. Bleed them in this order: right rear caliper, left rear caliper, right front caliper, left front caliper. Bleeding in this order will minimize the amount of air that gets into the system.

Repeat the process until you can no longer see any air bubbles coming out of any of the calipers. Make sure that you don't run out of brake fluid in your reservoir or you will have to start over again. It is wise to start out with about a 1/2 gallon of brake fluid in the pressure bleeder and another 1/2 gallon on the shelf in reserve. Depending upon your car, and the mistakes you may make, I recommend having an ample supply. Also, only use new brake fluid from a sealed can. Brake fluid is hydroscopic, meaning that it attracts water and water vapor, which diminishes its performance. Brake fluid containers left exposed to air will have the fluid inside compromised after a short period of time.

If you had to replace the master cylinder, or if the system needs a large amount of fluid, then supplement the bleeding process by opening up the right rear nipple and then pressing down on the brake pedal two or three times. Slowly release the pedal. Repeat for the other three corners of the car.

During the bleeding process, it's very easy to forget to check your master cylinder reservoir. As you are removing fluid from the calipers, it will be emptying the master cylinder reservoir. If the reservoir goes empty, then you will most certainly add some air bubbles in to the system, and you will have to start all over. Keep an eye on the fluid level and don't forget to refill it. Make sure that you always put the cap back on the reservoir. If the cap is off, then brake fluid may splash out and damage your paint when the brake pedal is released. If you are using a pressure bleeder system, make sure that you often check the level of brake fluid in the bleeder reservoir so that it doesn't accidentally run dry.

If you are installing a new master cylinder, it's probably a wise idea to perform what is called a brake bench bleed. This is the process of getting the master cylinder full of brake fluid and "wet." Install a brake bench bleed kit, mount the master cylinder in a vice, feed each hose into the reservoirs, add brake fluid to both reservoir chambers until the bleed hose is submerged and pump it until the bubbles stop. This can save you hours when bleeding brakes

Now, make sure that all the bleeder valves are closed tightly. Disconnect the pressure system from the reservoir. Follow the directions carefully and never loosen the reservoir cap when it is under pressure. Now, get your family member to repeatedly press the brake pedal down at least five times, and then hold it down. Take care not to push the brake pedal all the way to the floor, as you can actually damage the master cylinder by driving the piston into usually unused portion of the master cylinder and damaging the seals. Then open the bleeder valve on the right rear caliper. The system should lose pressure, and the pedal should sink to the floor.

When the fluid stops coming out of the bleeder valve, close the valve, and then tell your family member to let their foot off of the pedal. Do not let them take their foot off until you have completely closed the valve, or it will suck air back into the system. Repeat this entire procedure at least three time on each caliper in the same order as described previously.

I recommend that you use this procedure as a final step, even if you are vacuum bleeding or pressure bleeding. The high force associated with the pressure from the brake pedal can help free air and debris in the system. If the brake fluid doesn't exit the nipple quickly, then you might have a clog in your lines. Brake fluid that simply oozes out of the bleed nipple slowly is a clear indication that your rubber lines might be clogged and constricted. Don't ignore these warning signs, replace the brake lines while you are working in this area.

Now, let the car sit for about 10 minutes. Repeat the bleeding process at each corner. The pedal should now feel pretty stiff. If the pedal still feels spongy, make sure that you have the proper adjustment on your rear calipers or drum shoes. Also, you may need a new master cylinder, have a leaky caliper, or have old spongy flexible brake lines.

Another important thing to remember is that brake fluid kills paint jobs, that is. Brake fluid spilled on paint will permanently mar the surface, so be very careful not to touch the car if you have it on your hands and clothing. This, of course, is easier said than done. Don't bleed the system in a tight garage.

The probability of spilling on yourself and then leaning against your car is too great. Rubber gloves help protect yourself from getting it on your hands and your paint. If you do get a spot on your paint, make sure that you blot it with a paper towel--don't wipe or smear it, then quickly rinse with plenty of water. It's also important not to try to clean it off with any chemical or other cleaning solutions.

Make sure that you use DOT 4 brake fluid in your car.
Figure 1

Make sure that you use DOT 4 brake fluid in your car. The use of silicone DOT 5 fluid is not recommended for street use, and never mix DOT 4 and DOT 5 fluid together or brake failure can occur. Never use DOT 5 in an ABS system, unless it is designed for it or brake failure can occur. Shown here is the Motive Products Power Bleeder (red arrow) attached to the reservoir (yellow arrow) by its hose and cap. Available from, it is a huge timesaver when it comes to bleeding your brakes.

Open the bleed nipple by loosening it in the caliper by about a quarter of a turn (yellow arrow).
Figure 2

Open the bleed nipple by loosening it in the caliper by about a quarter of a turn (yellow arrow). If you can fit a flare-nut wrench over the bleed nipple, then I recommend using one, to help avoid rounding out the nipple. Let the brake fluid run out of the caliper until no more bubbles appear. You should use a clear tube (red arrow) so you can see the air bubbles escaping and when they stop. If you are working by yourself I always like to have a large drip pan or rubber container underneath my work area, brake fluid is very messy and will stain or ruin anything it comes in contact with. Mercedes-Benz recommends you flush and replace your brake fluid every two years. Deposits and debris can build up in the lines over time and decrease the efficiency of your brakes. Regular bleeding of your system can also help you spot brake problems that you wouldn't necessarily notice simply by driving the car. Also, never reuse brake fluidâÂÂÂÂ"always use new, fresh fluid. In addition, don't use brake fluid that has come from an open can that has been sitting on the shelf or sitting in your Power Bleeder for a while. The brake fluid has a tendency to absorb moisture when sitting on the shelf. This moisture "boils" out of the brake fluid when you start using the brakes and can result in a spongy pedal.

Comments and Suggestions:
DrFate Comments: After pressurizing the bleeder tank to only about 5-10psi, the reservoir on the master cylinder was leaking from the small rubber caps. Do you have any solution to this?
October 18, 2016
  Followup from the Pelican Staff: The maximum pressure you want to put the system under is 10psi. Try bleeding at 6 psi. I will have the article updated to reflect this info.- Nick at Pelican Parts  
Figgy Comments: When breaks is applied, the car will stop by applying extra force on the pedal. Previously the breaking pedal was very soft to apply fo the car to stop.
February 11, 2014
  Followup from the Pelican Staff: You may have air in the system or a faulty master cylinder. - Nick at Pelican Parts  

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Page last updated: Sun 2/18/2018 02:31:49 AM