[click to enlarge]
| 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.|
After talking to many owners, it would seem that there are more methods for bleeding brakes on a BMW than there are cures for the common cold. Fortunately, I have polled many people and tried several different solutions, and I think that I have come up with the best compromise solution. This article is adapted from some other articles that I have written on bleeding brakes on Porsches. The concepts are similar, and I have adapted them here along with pictures for bleeding the brakes on your BMW.
There are currently three methods of bleeding the brake system:
- Pressure Bleeding. This is where you have a reservoir of brake fluid, and place a positive air pressure force on the opposite side of the fluid, forcing it into the brake system.
- Vacuum Bleeding. This is where you fill the reservoir, and then apply a vacuum at the bleeder nipple to pull fluid through the system.
- Family Member Bleeding. This is where you recruit the one family member or friend who owes you a favor and have them stomp on the pedal repeatedly until the entire system is bled. Note that this has nothing to do with the time that little Jimmy fell on the concrete and had to be rushed to the hospital.
The method that I've come up with combines the first and the third methods described above. Basically, I advocate bleeding the system with the pressure bleeder, and then using a family member to stomp on the pedal to free up the proportioning valve. If the family member really owes you big time, you will be the one stomping on the pedal, and they can spill brake fluid all over themselves.
| Now start bleeding the system. Start with the right rear caliper, the one that's located furthest away from the master cylinder. You should remove the rear wheels of the car to easily get to the rear caliper (Figure 3). The whole process is a heck of a lot easier if the car is off of the ground, and the wheels have been removed. The front wheels can be turned for access to the calipers. Bleed the right rear caliper by attaching a hose to the bleed nipple, placing it in a jar, and then opening the valve with a wrench (a 7mm wrench is typically needed). A bleeder nipple is shown in Figure 4, and can be opened by turning it counter clockwise. Let the fluid out until there are no more bubbles (Figure 5). 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 stomp on 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 bleed the system completely. If your rear caliper has two bleed nipples (some have one, others have two), bleed the lower one first.|
When no more air bubbles come out, then 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 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, it's wise to have an ample supply.
During the bleeding process, its 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 dont 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 you dont accidentally run dry.
If you are installing a new master cylinder, its probably a wise idea to perform what is called a dry-bleed on the workbench. This is simply the process of getting the master cylinder full of brake fluid and wet. Simply add some brake fluid to both chambers of the master cylinder, and pump it a few times. This will save you a few moments when bleeding the brakes.
Now, make sure that all the bleeder valves are closed tightly. Disconnect the pressure system from the reservoir. Now, get your family member to press down repeatedly on the brake pedal at least five times, and then hold it down. 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. Repeat this motion for each bleeder valve on each caliper at least three times. Repeat this entire procedure for all the valves in the same order as described previously.
I recommend that you use this procedure as a final step, even if you are vacuum or pressure bleeding. The high force associated with the pressure from the brake pedal can help free air and debris in the lines. If the brake fluid doesnt exit the nipple quickly, then you might have a clog in your lines. Brake fluid that simply oozes out of the lines slowly is a clear indication that your rubber lines might be clogged and constricted. Dont ignore these warning signs check out the brake lines while you are working in this area.
Then, 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 then done. Just be aware of this fact. Rubber gloves help to 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 - dont wipe or smear it. Its also important not to try to clean it off with any chemical or other cleaning solutions.
| There are few little tricks that you can use when changing your brake fluid. The company ATE makes a brake fluid called SuperBlue that comes in two different colors (Figure 6). Its a smart idea to fill your reservoir with a different colored fluid, and then bleed the brakes. When the new colored fluid exits out of the caliper, you will know that you have fresh fluid in your system. Make sure that you use DOT 3 or DOT 4 brake fluid in your car. Some of the later model BMWs with anti-lock braking systems require the use of DOT 4. The use of silicone DOT 5 fluid is not recommended for street use.|
You should also routinely 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 wouldnt 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 empty can that has been sitting on the shelf. 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 results in a spongy pedal.
Well that's about all it takes. If you would like to see more technical articles like this one, please continue to support Pelican Parts with all your parts needs. Your continued support directly affects the expansion and existence of this site and technical articles like this one. As always, if you have any questions or comments about this helpful article, please drop us a line.