While parts of the of the ABS system can be removed and replaced by the owner of the vehicle, special tool, technique and procedure are needed to check the operation and performance of the system once repairs are completed.
To bleed the hydraulic system, a necessary step after the hydraulic system has been repaired, the BMW Diagnostic Tester or equivalent must be utilized to electronically open the internal valve. Only this way can the system be completely bled.
To check the operation of the ABS system after any of the electronic portions have been replaced or disconnected, the BMW Diagnostic Tester or equivalent must be used.
It is recommended to allow a licensed and trained professional to complete repairs on the ABS system. Most repairs are straightforward, but the diagnosis and testing of the system can enter a different realm. The safety and integrity of the braking system must never be compromised.
To ensure continued proper operation of the braking system, the brake fluid must be changed at least every 2 years. If the vehicle is used for heavy duty use, change the fluid at least once a year. If the vehicle is used in competition or drivers schools, bleed the system before and after each event or at least every 90 days.
Use a quality, approved DOT 4 brake fluid. Use fluid from sealed, unused cans. Buy the fluid in quantities that you expect to use. Buying a large container of fluid and only using a small portion can lead to the remaining fluid to become contaminated over time. Brake fluid absorbs moisture and can lead to system corrosion.
- Fill the master cylinder to the maximum level with the proper brake fluid.
- Attach a pressure bleeder to the master cylinder reservoir. Do not allow the pressure to exceed 29 psi (2 bar).
- Raise and safely support the vehicle.
Remove the protective caps from the bleeder screws and bleed the brakes using the following bleeding sequence. Always starts with the brake unit farthest from the master cylinder. The proper bleeding sequence is:
- Insert a tight fitting clear plastic tube over the bleeder screw on the caliper and the other end of the tube in a transparent container partially filled with clean brake fluid.
Have the assistant apply pressure to the brake pedal.
- Open the bleeder.
- While the bleeder is opened, have the assistant pump the brake pedal slowly at least 12 times.
- When the fluid being dispersed is free of all air bubbles, hold the brake pedal down and close the bleeder valve.
- Slowly release the brake pedal.
- Check and top off the brake fluid reservoir and pressure bleeder as necessary.
- Torque the 7mm bleeders to 44 inch lbs. 3.5 ft. lbs. (5 Nm) and the 9mm bleeders to 53 inch lbs. 4.3 ft. lbs. (6 Nm). Replace the bleeder dust covers.
Repeat the brake bleeding steps above for each caliper in the following order.
- Left rear
- Right front
- Left front
- Lower the vehicle, check the brake fluid level, top off as necessary and check for normal operation of the brakes.