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Brake Bleeding and Flushing

Pelican Technical Article:

Brake Bleeding and Flushing

Tom Morr


1 hour1 hr


1 Liter of Brake Fluid ($12.25-$17.25


Line wrenches, combination wrenches, ratchet/sockets

Applicable Models:

Mercedes-Benz R170 (1998-04)

Parts Required:

Brake fluid

Hot Tip:

A pressure bleeder makes it a one-person job

Performance Gain:

Restores brake performance
Many Mercedes-Benz owners shy away from hydraulic-system maintenance. It can be messy, and these systems are typically safety-related (brakes and steering). However, no special expertise is involved, so any R170 owner with basic mechanical skills and patience can bleed their brakes.

Any brake job that allows air to enter the lines obviously requires bleeding the system. However, brake fluid should be completely flushed/replaced every two years, even if air hasn't infected it. Condensation in the brake lines, calipers, and master cylinder introduces moisture into the fluid, minimizing its effectiveness.

Having the brake fluid professionally bled and flushed is obviously easiest. More affordable is doing it yourself. Assuming that your brake system doesn't have any other issues, bleeding/flushing can be accomplished in about an hour.

The traditional two-person method requires one person in the car to gingerly "pump up" pedal pressure, then keep the pedal depressed while someone else opens the bleeder screw and captures evacuating brake fluid in a bottle or other suitable container. This cycle is repeated, sometimes seemingly ad infinitum, until pedal pressure is firm and clean, bubble-free fluid comes from each caliper.

For many enthusiasts, one-person power-bleeding is a better solution. It involves buying a bleeding system, which can be less than or comparable to having the dealership or repair facility professionally bleed the brake fluid once. Power bleeders range from hand-pump units to higher-end air compressor-actuated models. Pelican Parts sells the compressor-powered Mityvac system (Part No. MTVC-001). It isn't the least expensive option, but it produces quality one-person bleeding results.

Standard glycol-based brake fluid is corrosive. It makes paint bubble. Have rags and a squirt bottle of water handy. Disperse and clean up any drips or spills promptly. Wear gloves and glasses to protect human body parts from brake fluid. When doing the old-school two-person pump/crack method, some enthusiasts put a 2x4 block under the brake pedal to help maintain constant pressure and prevent piston damage.

A one-person manual bleeding option is Speed Bleeders, available from Pelican Parts for many MB models. These replacement bleeder screws have an internal check valve that allows air and fluid to escape, but no air to re-enter. 

Pressure-style power bleeders apply air pressure to the master cylinder to push fluid through the lines and out the bleeder. Vacuum bleeders pull fluid out through the caliper's  bleeder screws.

Spray penetrating lubricant on the bleeder screws before attempting to unscrew them.
To avoid rounding off the bleeders, use a line wrench, box wrench, or a 12-point socket. The R170 SLK takes a 9mm wrench.

A small amount of fluid remains in the ABS pump. Dealerships often have a special bleeding machine that flushes this fluid also. Many owners find the amount of trapped fluid negligible. Those who are concerned about this can flush  at normal recommended service intervals, drive and actuate the ABS a few times to rotate the trapped fluid, then flush again.

Alternating brake fluid colors (blue and gold are popular) at every flush will help reveal when the old fluid is completely flushed. 

Moisture in ambient air will gradually degrade brake fluid once the bottle is opened. Ideally, buy small bottles of fluid, and always use fluid from a sealed container. Discard unused fluid.

M-B recommends DOT 4 fluid for the R170 SLK. If you participate in track days, you might want to research boiling points of various manufacturer's DOT 4 fluids and go with one of the higher-rated brands.

Start at the caliper the farthest away from the master cylinder and get progressively closer. For the R170 and many other cars, the sequence is right rear, left rear, right front, left front.

Some models, such as the R140, also have a bleeder on the master cylinder.

Check the master cylinder often during bleeding/flushing to make sure that it doesn't go dry and allow air to enter the lines. If the pedal feels spongy, keep bleeding. The manual bleeding method can require 20 or more strokes of the pedal per corner to pass all air and old fluid. If the pedal doesn't feel right after exhaustive bleeding, air could be entering the lines. Look for leaks, particularly at the lines' and hoses' connections.

Brake fluid capacity is .45-.6 liter. The typical flush requires about .75 liter. The bleeder screw torque spec is 7 Nm / 5 ft-lb. Make sure you dispose of old brake fluid properly.
One of the easiest, most reliable one-person bleeding systems is the vacuum-style Mityvac MV6830 bleeder, available from Pelican Parts (MVTC-001).
Figure 1

One of the easiest, most reliable one-person bleeding systems is the vacuum-style Mityvac MV6830 bleeder, available from Pelican Parts (MVTC-001). It can also be used to bleed hydraulic clutch cylinders.

Residual pressure in the brake booster normally dissipates when the car is shut off.
Figure 2

Residual pressure in the brake booster normally dissipates when the car is shut off. To make sure, gently pump the pedal a few times with the car off.

Removing old fluid from the master cylinder expedites flushing.
Figure 3

Removing old fluid from the master cylinder expedites flushing. A turkey baster, rags, and a cup are the traditional method. Higher-tech is using the Mityvac vacuum bleeder with its suction-tube attachment. Stirring the fluid with the tube helps dislodge sludge. Leave enough fluid in the bottom of the master cylinder so air won't enter the lines.

The Mityvac kit includes a fluid refill kit: various master cylinder plates and spouts that screw onto brake fluid bottles.
Figure 4

The Mityvac kit includes a fluid refill kit: various master cylinder plates and spouts that screw onto brake fluid bottles. This keeps the master cylinder topped up during bleeding. Alternately, check the level frequently to ensure that the master cylinder doesn't run dry.

The job can be done with the tires on the car.
Figure 5

The job can be done with the tires on the car. Raising and securing the car eases access to the bleeder screws. To guard against bleeder-screw stripping, spray the threads with penetrating spray.

Use a line wrench, box wrench, or socket to crack the bleederâ€
Figure 6

Use a line wrench, box wrench, or socket to crack the bleederâÂ"open-end wrenches tend to round off bleeders' shoulders, creating more work. Begin with the right rear corner or the one that's farthest away from the master cylinder. (Our left rear was the most photogenic.)

Various-sized bleeder boots are included in the Mityvac kit.
Figure 7

Various-sized bleeder boots are included in the Mityvac kit. Select the one that fits snuggest and install it on the vacuum hose's barbed end. Then crack the bleeder screw a half turn or more with an open-end wrench. The Mityvac unit has a throttle on its handle that turns air pressure from a compressor into vacuum force. The kit also includes a hook for hanging the reservoir.

Bleed until fresh fluid comes out.
Figure 8

Bleed until fresh fluid comes out. Mityvac's instructions list 20-30 seconds as typical bleeding time. Unlike manual bleeding, air bubbles will be visible in the hose throughout: Vacuum pressure sucks air through the bleeder screws' threads. If this is objectionable, Mityvac recommends coating the bleeder threads with silicone grease to form a seal. Once fresh fluid appears, close the bleeder screw and move to the next-farthest-away-from-the-master-cylinder caliper.

After bleeding, clean up any residual fluid.
Figure 9

After bleeding, clean up any residual fluid. Turn on the car, gently pump the brakes, and check for leaks. Then test-drive the car. If the pedal feels spongy or has excessive travel before the brakes grab, repeat the bleeding procedure.

Comments and Suggestions:
Johne Comments: Well from a driveable car with spongy brakes to a mechanical disaster. I took the ABS pump motor off to check and claen and the bearing came out with it. So of course I had to remove the ABS unit to get it back in. Fine got the pump working and put everything back on the car, Bled brakes, still spongy ABS etc lights on now. Took for a drive to do some ABS stops and found speedo not working and stuck in first gear. Feeling very dejected.
March 5, 2017
  Followup from the Pelican Staff: What vehicle are you working on? Are there any ABS fault codes stored? The ABS unit may have been damaged.- Nick at Pelican Parts  
Johne Comments: Thank you for your help. I gave the brakes another bleed and a good run with lots of hard stops. It stops on a dime but still spongy. Another bleed to try and repeat. I am going to put a new ABS hydraulic system on my wish list. Does pelican actually supply these or can they be overhauled? Thanks
March 1, 2017
  Followup from the Pelican Staff: I am not sure if they can be overhauled. I do not see it offered in our catalog. If you give our Mercedes parts specialist a call with your VIN number we can get one from MBZ. 888-280-7799 - Casey at Pelican Parts  
Johne Comments: Thanks for that, I have used both vacuum and pressure bleeding and the 2 person method. The car does stop well but the pedal is too spongy for my liking. I have been wondering about the ABS unit. Is it possible to flush that separately, I thought it would have been flushed with the system bleed but it soubds like there is another level of bleed I hadn't known of. I suppose a new ABS unit is the next phase.
February 27, 2017
  Followup from the Pelican Staff: You usually need to use a scan tool to bleed the ABS unit, but I have been able to do this by performing a few ABS stops, bleeding the calipers, then performing a few more stops, bleeding the calipers and continuing 4 times or so, if you see no improvement probably time to replace the ABS hydraulic unit. Obviously make sure you are not going to get rear ended during your ABS stops! - Casey at Pelican Parts  
Johne Comments: Bought a new Mertelli Master cylinder. I have a pressure system a vacuum system and even one way valves to use on the end of the drain hose. Also tried 2 person bleeding. Bled until lines show no air but still a spongy pedal which goes to the floor. Air must be trapped somewhere. Can see no leaks anywhere. This was the same problem that lead me to buy the new M/cylinder. pedal goes to the floor if pressure kept on it.
February 24, 2017
  Followup from the Pelican Staff: How does the car stop?? I find that diagnosing a brake master cylinder should be done by how the car stops, not how the pedal feels. The vacuum assist can make a good cylinder feel bad until you actually stop the car and notice that that light pedal doesn't need to make the full sweep of travel to stop the car... Let us know! Also might be a good idea to have the ABS unit flushed in case you have some old fluid stuck in there. Just came to mind that the ABS solenoids can bypass fluid causing the same concern. Your problem may live inside the ABS hydraulic unit. - Casey at Pelican Parts  
Gman Comments: Does a spongy brake pedal indicate that new fluid and bleeding system is required? Car has not been driven in over 6 months. Will brake fluid seep out of system if no used regularly. In other words,does lack of use cause problems with brake systems?
May 7, 2016
  Followup from the Pelican Staff: Spongy pedal is more likely a faulty master cylinder, brake line or a leak. - Nick at Pelican Parts  

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Page last updated: Sat 2/24/2018 02:36:06 AM