Introduction

BMW recommends that the brake fluid system in their cars be flushed and bled every year. Considering the importance of properly working brakes, this is the single most important maintenance item and often the most overlooked. The primary threats to a brake system are water, dirt, and air. Brake fluid will absorb water and rust out the system from within. It will also accumulate dirt, which can ruin the machined surfaces in the brake master cylinder and the brake calipers. And air can sit in the lines and cause the brakes to be spongy and less effective.

How often should you flush and bleed your brakes? Well, too much flushing your brakes is like too much sex - there's no such thing. But seriously, for most applications, once a year is adequate. But it is also dependent upon the amount of driving you do per year, the amount of city versus highway driving you do, what kind of transmission your car has, and how you use your brakes. The amount of money spent on brake fluid is negligible (around $10-15, at most), so this shouldn't be a consideration.

For BMW owners attending performance driving schools, it is essential that this service be performed competently and more frequently. Therefore, many of us do-it-yourself mechanics prefer to do this service ourselves. While traditionally a two-person job (one at the caliper, one in the driver's seat pumping the pedal), it is helpful to be able to bleed brakes without the assistance of another. While I have spent time on the weekend prior to a school servicing my brakes, I have witnessed some driving school attendees bleed their brakes at a school between track sessions in remarkably short order.

A pressure bleeder can be an essential tool in this job. Due to the way a pressure tool works, it is imperative that a sufficient vacuum is achieved. There are a number of commercial bleeders available on the aftermarket today. These use the principle of sucking the fluid out of the caliper. Unfortunately, general consensus is that they don't work very well. And amongst my friends, we've tried all the ones you've read about. Therefore, Al Cote and Jon Lindsay came up with a homemade tool which pressurizes the system and pushes the fluid out of the caliper; it works quite well.

This FAQ is broken up into two parts. In the first part, I give some general guidelines on how to bleed your brakes. In the second section, you'll find the bleeder parts list and some associated instructions. Please remember that you should consult your repair manual for any unique features about your BMW that should be considered while performing this service. Examples of these unique features are consideration of the ABS pump, and whether the car has a pressure accumulator (or bomb).

Bleeding Brakes

1. Some folks like to replace the fluid in the reservoir prior to flushing the lines. Some have been insensitive enough to use kitchen utensils (like turkey basters) for this task. This has been known to cause family unrest, and the author of this FAQ suggests you find an alternate solution for the sake of your family. Truly, unless the fluid is excessively dirty (very dark), this really isn't necessary.
2. Assemble the bleeder as discussed by Al and Jon below.
3. Put about a half quart of your favorite fluid into Tank #1 and tighten the lid. Refer to your owner's manual for DOT specification if you are unsure. Unless you are a driving school veteran and are attending more than 10 track days per year, I don't think it is necessary to waste your money on racing fluids.
4. Replace cap on the reservoir with Cap #3 and make sure you have a good seal.
5. Pressurize Tank #1 to 10-15 psi. Check for leaks.
6. Start with the caliper closest to the brake fluid reservoir. You might step on the brake pedal once or twice to get the fluids moving.
7. With the brake line closest to the fluid reservoir, open your caliper nipple. With the first line, you want to completely replace the fluid in the reservoir with fresh fluid (unless you did this prior to beginning). Close the nipple when the fluid is very clean and when you see no more air bubbles.
8. Work your way towards the longest brake line's caliper and repeat this procedure.
9. For completeness, I like to repeat the procedure again to make sure there is no more air trapped in the system. If you have replaced brake lines or rebuilt calipers, and have introduced air into the system, it is imperative that you repeat the procedure. Otherwise, use your best judgement.
10. One other note is that it has been suggested that one start with the line furthest from the fluid reservoir, but until someone can give me conclusive evidence explaining why this works better, I'll stick with this for now.
11. After you are finished, release the pressure in the tank by pushing down on the valve release... then remove the caps.

Brake Bleeder Parts List

courtesy of Jon and Al

1. TANK -- 1/2 gallon heavy duty (Nalgene) plastic tank. A dealer of outdoor equipment should easily have such a tank. They have many different styles and shapes. Your criteria should be a tight seal on the screw cap, very solid construction, and a relatively flat surface area on the top where you can mount a metal tire valve. We used a tank with the following dimensions: 8"(h)x6"(w)x3"(d). It is most important that it be sturdy and that most of the volume be air rather than fluid so that the pressure remains relatively constant during the bleed. Use a good DOT-approved fluid and put about a half quart in the tank. This tank was the most expensive part ~ $9.50
2. TANK -- 1quart tank into which old fluid is collected. You have probably used a form of this in the past. At the same same outfitter store, you can get a lighter duty quart jug for this purpose. Put another tire valve in the cap, drill the valve out with a 1/4" bit, and run a 5/16" hose from the caliper nipple over a short section of stiff 1/4" tubing which goes through the valve and down into the tank. A very small hole drilled into the cap next to the valve will allow air to escape. No more catching fluid in a wine bottle, if this one falls over it is no problem, because it is a single piece.
3. CAP -- Nalgene cap to temporarily fit the top of the reservoir in place of the existing one with the sending unit. Find a Nalgene cap about 1 3/4" in diameter to fit your BMW perfectly (a tight fit here is essential). This part may take some trial and error and the cap from one reservoir may be different from another.
4. HOSE -- 2 feet of 3/8" OD x 1/4 ID vinyl hose-- to go from Tank #1 to the cap #3.
5. HOSE -- 2 feet of 5/16" OD hose to drain the fluid into Tank #2.
6. HOSE -- 9" of stiff 1/4" OD tubing to fit inside Tank #1 from the drilled valve down to a corner in the tank (take a look at the pesticide tank in your garage if you can't visualize this).
7. VALVE STEMS -- 4 metal type screw valve stems -- these should be available at most car parts or tire stores. Drill out three of them as described for Tank #2 above. The fourth one is not drilled out because it is mounted in the cap of #1 above and is used to pressurize the system.

Bleeder Construction

courtesy of Jon and Al

1. Drill out three of the four valves with 1/4" bit (be careful and use a vise).
2. Drill out all three caps to accept the valves as well as a spot on the shoulder of Tank #1. Mount undrilled valve in cap of Tank #1. Insert 1/4" stiff tubing into bottom end of one remaining valve and mount valve on shoulder of Tank #1. Mount a remaining drilled valve into the hole in cap which mounts on reservoir. Put 3/8" OD hose on the two valve stems just described.
3. Collector tank construction is described above. You can use a small electric pump but a hand pump will do. Don't pressurize above 10 to 15 psi.
4. Larger hoses will improve flow and a stop cock valve allows you to fine tune your setup but is not necessary. A local observer suggested a strap for the reservoir to ensure your reservoir doesn't decide to lift off, another unnecessary precaution in my experience.