How to Bleed ABS Brakes

Knowing how to bleed ABS brakes is a required skill for proper maintenance-brake fluid should be replaced every two years and after any brake repair. Brake fluid can be compromised by air, water or debris-bleeding the lines replaces the old brake fluid with new, clean fluid.

The Antilock Brake System (ABS) modulator is the device that controls the application of breaking power in an emergency breaking event. This complex unit sits between the master cylinder and the rest of the braking system. ABS modulators have between 8 and 10 valves, making it impossible to effectively bleed these units without the use of specialized equipment.

Bleeding ABS Brakes Safely

If you are repairing or replacing components below the modulator, than standard bleeding technique can be used. Repairs to the modulator or upstream components, however, require a different technique. Here's how to tackle it:

• Go by the book. Don't even consider bleeding ABS brakes without first consulting a service manual for your particular vehicle. Each ABS system has its own procedures and quirks and the only way to have complete knowledge is with a service manual.
• Down below. For repairs down stream of the ABS modulator (brake caliper, rotors, pads, etc.), normal bleeding techniques can be used. Manual, pressure or vacuum bleeding can all be used, as long as all air is removed from the lines and fresh brake fluid has completely replaced old fluid.
• The big guns. A pressure bleeder and scan tool will be required to bleed the ABS modulator if air has entered the system at or above the unit. A pressure bleeder attaches to the master cylinder reservoir and is used to pump brake fluid into the system. A scan tool is an electronic device that can communicate with the computer processor that runs the ABS modulator.
• High tech bleeding. The scan tool will cycle through the solenoids that control each valve of the BAS modulator as the pressure bleeder forces brake fluid through the system. After the modulator has been purged, the scan tool will direct you to bleed each brake at the wheel until all air is removed from the system. The scan tool will then run a final of the ABS solenoids to purge any remaining air.