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Pelican Technical Article:

Brake Bleeding

Kerry Jonsson


1 hour1 hr






11mm wrench

Applicable Models:

Mercedes-Benz R129 (1990-02)

Parts Required:

Brake lines

Hot Tip:

Put a tie wrap around the hose on the bleeder screw to prevent it from falling off

Performance Gain:

Remove old, contaminated brake fluid.

Complementary Modification:

Replace brake pads at the same time
As you step on your brake pedal a piston in your master cylinder compresses a piston filled with brake fluid. As this fluid gets compressed it is forced through lines and hoses to the caliper and causes the caliper pistons to act on the brake pads. The friction between the brake pad and the brake rotor is what slows down the car. This generates a lot of heat that needs to be absorbed by the brake rotor, the brake pad and then eventually ends up at the brake fluid. The quality measurement of brake fluid is its boiling point. If your brake fluid were to get so hot it started to boil it would expand and drive the pistons into the brake pads, therefore apply some brakes even without your foot being on the brake pedal. This, in turn, generates more heat and can lead to brake component failure. A quality racing brake fluid can go up to 650 degrees to 750 degrees Fahrenheit. A conventional commercially available brake fluid would be lower than that. Brake fluid is hygroscopic which means it absorbs moisture out of the air. This, in turn, lowers the boiling point of the brake fluid. Over time brake fluid wears out and contaminants start to become suspended in the brake fluid. This can cause calipers to stick and ABS computer solenoids to stick. In this tech article we will go over all the steps to flush out your brake system.

Now there are a few ways you can bleed your brake system depending on the level of equipment you have. The best way and fastest way to bleed the brakes are pressure bleeders. These pressure bleeders are filled with brake fluid. You add air to a bladder on the other side of the brake fluid diaphragm and this pressurizes the brake fluid. You attach this pressurized brake fluid, through adaptors, to the brake master cylinder fluid reservoir. Now pressurized brake fluid is being applied to the brake system of the car. All you have to do is open the bleeder screw and old brake fluid will be forced out. Keep the line open until you see clean brake fluid coming out of the bleeder screw without any air bubbles in it. There are two types of these pressure bleeders; one uses shop air to generate pressure and one has a manual pump built into it. We have an air powered pressure bleeder and that is what we are going to show you.

A second type of powered bleeding system is the vacuum type bleeder. Here you attach a vacuum pump to the bleeder screw on the caliper. There are two types of vacuum generating bleeder systems. One uses compressed shop air if you have a compressor the other has a manual pump that generates the vacuum. Make sure the reservoir is full and apply a vacuum to the bleeder screw then open the screw. The tool will draw the old brake fluid out and it will be replaced with the new fluid you put in the master cylinder reservoir.

Finally the last method requires a helper. The helper will pump on the brake system and finally hold it down to generate the necessary brake system pressure. While they are pushing the pedal to the floor you open one of the bleeder screws. Fluid and air will come out and the brake pedal will sink to the floor. When the brake pedal hits the floor have your helper hold it there until you close the bleeder screw. Have your helper release the brake pedal then pump it up 2 to 3 times again. Open the bleeder screw again and repeat this step at the same brake caliper several times until the brake fluid runs clear and is free of air bubbles. Start at the right side rear wheel, followed by the left side rear wheel, followed by the right side front wheel and finally the left side front wheel. Make sure the brake fluid reservoir is topped off each time you switch to bleed a different wheel.

Mercedes Benz recommends you replace your brake fluid every two years. In order to bleed the brakes you should lift and support your vehicle. It will be more difficult on the ground where you have to reach behind the wheels while you open and close the brake caliper bleeder screw. It is possible but makes the job more difficult. See our tech article on jacking up and supporting your vehicle.
This picture illustrates the engine compartment on the left side by the base of the windshield.
Figure 1

This picture illustrates the engine compartment on the left side by the base of the windshield. Clean the area around the master cylinder before opening the cap to prevent any dirt from getting in it. Attach you pressure bleeder to the brake master cylinder fluid reservoir. Make sure it is tight so brake fluid does not leak out. Also make sure the flow valve (green arrow) is in the open position.

Pressurize the brake bleeding tool to 10psi, do not exceed 10 psi.
Figure 2

Pressurize the brake bleeding tool to 10psi, do not exceed 10 psi. Here our tool is air powered so we apply compressed air by letting air into the chamber filled with clean brake fluid.

First install a 9mm line wrench (yellow arrow) on the caliper bleeder screw then put the drain hose on the bleeder screw nipple (green arrow).
Figure 3

First install a 9mm line wrench (yellow arrow) on the caliper bleeder screw then put the drain hose on the bleeder screw nipple (green arrow). Start at the right rear wheel first, followed by the left rear wheel, followed by the right front wheel, which is where we are, and finally the left front wheel.

Open the bleeder screw and allow old brake fluid to flow into a drain pan.
Figure 4

Open the bleeder screw and allow old brake fluid to flow into a drain pan. Watch the brake fluid until it starts to look cleaner (new fluid) and it has no air bubbles in it then loosen the bleeder screw. Before moving on to the next wheel be sure the reservoir is full of brake fluid from your pressure supply tool. When you are done tighten all bleeder screws and install their protective caps to prevent air from getting in creating corrosion in the bleeder screw. Corrosion would make the bleeder screw harder to loosen and the screw's head may even get stripped out.

Comments and Suggestions:
McGee Comments: I forgot to add that my car is a 1992 Mercedes 300SL and the ABS pump is part number 0 265 200 043.The car has 125K miles on it.
August 31, 2017
  Followup from the Pelican Staff: Got it, thanks. - Nick at Pelican Parts  
McGee Comments: No Front Brakes.Car has been sitting in doors for quite some time.Plenty of fluid and pressure from the master cylinder to the ABS unit but nothing from the ABS fittings very loosened to the front wheel lines.Tried it with the engine running and off.Open the front bleeders on the calipers,not even a drip.Rear brakes are working.Any ideas ?Thanks
August 31, 2017
  Followup from the Pelican Staff: The ABS system likely needs bleeding using a Mercedes-Benz scan tool. if that doesn't do it, the hydraulic unit may be faulty.- Nick at Pelican Parts  
Ax Comments: Thank you for the article - really helps. In second sentence - do you not mean to say "tighten" bleeder screw?
December 23, 2015
  Followup from the Pelican Staff: I can't find in the article where you are referring to, can you give me the step that sounds like it has a typo? - Nick at Pelican Parts  
Bruno Comments: Hi, really enjoying your articals for my MB SL320 W129 1995. That said, you have a double entry for bleeding the brakes.ARTICLE- Brake Rotor Replacement shows brake bleeding. Hope you can resolve and upload correct version. I need it ASAP. thanks. Bruno UK
August 16, 2014
  Followup from the Pelican Staff: I was able to navigate to the article. The article is located here:

- Nick at Pelican Parts

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Page last updated: Mon 1/15/2018 02:41:38 AM