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Pelican Technical Article:

Bleeding Brakes

Steve Vernon

Time:

2 hours2 hrs

Tab:

$20

Talent:

**

Applicable Models:

Mercedes-Benz W124 (1986-95)

Parts Required:

Three + 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 the 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 different colored brake fluid, in order to determine when fresh fluid has been flushed through the entire system. 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 25 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.

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 an 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 times 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 (yellow arrow) attached to the reservoir (red arrow) by its hose and cap. Available from PelicanParts.com, it is a huge timesaver when it comes to bleeding your brakes.

You should have and use a 9mm flared nut wrench to open the bleed nipples.
Figure 2

You should have and use a 9mm flared nut wrench to open the bleed nipples. The wrench grasps the nipple with the wrench more completely than a standard wrench and helps open the nipples without rounding them off. The nipples can get stuck with all the road grime so make sure you use the right tool and eliminate the chance of damaging them.

Open the bleed nipple by loosening it in the caliper by about a quarter of a turn (yellow arrow).
Figure 3

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


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Comments and Suggestions:
rontv Comments: Anyone can assist me in getting the air conditioner to work on my 1993 Mercedes Benz 300E. Every year I had to recharge the AC system, but last year it just will not take a charge. I used the gauges and tried to vacuum the system, but it will not take the vacuum. I tried to find where the leak is with dye, but nothing is showing up. I have not gone down to the evaporator since it is very difficult to get to. Any suggestion?
June 14, 2016
  Followup from the Pelican Staff: You likely have a large leak. I would have a professional use a leak detector to locate the source. - Nick at Pelican Parts  
rontv Comments: I changed my front brakes several years ago by myself and the brakes work okay all these years. Last week I purchased some new brakes and was about to do it again myself, but couldnt find the time, so at 11;am I took it to a neighborhood garage for them to do it for me. They had the car all day and at 6:pm after work I went to get it and almost ran into their garage door because the brakes pedal went all the way to the floor before the car can stop. I saw the mechanic jacked up the front of the car and he tried to bleed the brake with another mechanic inside the car pumping, but he could never get any pressure. He even unscrewed the bleeding plugs all the way out and had them in his hands, but no fluid was even coming out the bleeding holes. I got so mad I took the car home and tried to bleed the brake myself starting from the right rear wheel. I am getting good bleeding pressure from the rear, but nothing at all at the front. What could he have done wrongly? I cant drive the car now because I have no brake at all at the front and the brake pedal goes all the way to the floor. Any suggestion? Oh, its a 1993 Mercedes Benz 300E ABS Brakes.
June 11, 2016
  Followup from the Pelican Staff: The ABS unit may have air in it. I would try activating the ABS pump and valves using a Mercedes-Benz scan tool. That may bet the air out. - Nick at Pelican Parts  
RonTV Comments: Im sorry to be specific, how to do a gravity bleeding for a few minutes on 1993 300E ?
June 9, 2016
  Followup from the Pelican Staff: Remove brake fluid cap. Then open bleeders and allow the fluid to drain. - Nick at Pelican Parts  
RonTV Comments: How to do a gravity bleeding for a few minutes?
June 9, 2016
  Followup from the Pelican Staff: Remove brake fluid cap. Then open bleeders and allow the fluid to drain. - Nick at Pelican Parts  
oldtrucker Comments: Forgive me for asking this Q. Does a ABS system go by the same bleeding process? I read somewhere that the ignition should be on. Is this valid or does it not matter, ignition on or off?
Thanks!
June 9, 2015
  Followup from the Pelican Staff: Depends on the vehicle. Some it is the same, some the ABS system needs to be bled using as scan tool. - Nick at Pelican Parts  
Dave Comments: Thanks for the response.

My wife, actually my girlfriend then, once helped me with the standard pumping method. Once. I tried gravity feeding one Thanksgiving, and was still waiting on Christmas! Don't forget to tip your server!

Seriously, I wound up taking a tube and connecting it to the two valves, thus closing the system. Worked well.
December 19, 2014
  Followup from the Pelican Staff: Awesome, Thanks for the follow up. We appreciate it.
- Nick at Pelican Parts
 
Dave Comments: I tried the pressure method with a Motive Pressure Bleeder on my 1986 300e. There are two rubber caps on the brake fluid reservoir on my car, and underneath these are valves to let air into the system. Unfortunately, these valves prevent the pressure bleeder from building pressure and renders it useless. I am still trying to figure out a solution, so any suggestions are welcomed. On other cars air is let in through a hole in the reservoir cap, and since the cap is replaced by the pressure bleeder's cap with hose when bleeding, it is not a problem.
December 17, 2014
  Followup from the Pelican Staff: You can stick to manually bleeding. First let it gravity bleed for a few minutes, then use the standard pedal pumping method. - Nick at Pelican Parts  

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