See Figure 1
A Diagnostic Readout Box (DRB), or the equivalent type of scan tool, is essential to perform the bleeding of the ABS system.
The brake system must be bled any time air is permitted to enter the system through loosened or disconnected lines or hoses, or anytime the modulator is removed. Excessive air within the system will cause a soft or spongy feel in the brake pedal.
When bleeding any part of the system, the reservoir must remain as close to full as possible at all times. Check the level frequently and top off fluid as needed.
This is basically a three step process consisting of: a conventional manual bleeding procedure, a second bleeding procedure using a DRB, followed by a repeat of the first bleeding procedure.
- Clean around the reservoir cap and remove the cap. Fill the master cylinder with the recommended fluid.
- The recommended bleeding sequence is: master cylinder, HCU valve body (at the fluid lines), right rear wheel, left rear wheel, right front wheel and left front wheel.
- Clean all the bleeder screws. You may want to give each one a shot of penetrating solvent to loosen it; seizure is a common problem with bleeder screws, which then break off, sometimes requiring replacement of the part to which they are attached.
Brake fluid absorbs moisture from the air. Don't leave the master cylinder or the fluid container uncovered any longer than necessary. Be careful handling the fluid-it will damage the vehicle's paint.
Check the level of the fluid often when bleeding, and refill the reservoirs as necessary. Don't let them run dry, or you will have to repeat the process.
- Attach a length of clear vinyl tubing to the bleeder screw on the wheel cylinder. Insert the other end of the tube into a clear, clean jar half filled with brake fluid.
- Have your assistant slowly depress the brake pedal. As this is done, open the bleeder screw until the brake fluid starts to flow through the tube. Then, close the bleeder screw when the brake pedal reaches the end of its travel. After the bleeder valve is fully closed, have your assistant slowly release the pedal. Repeat this process until no air bubbles appear in the expelled fluid.
- Repeat the procedure on the other three brake cylinders/calipers, checking the lever of brake fluid in the master cylinder reservoir often.
Perform the "Bleed Brake'' procedure with the DRB scan tool:
- Engage the DRB scan tool to the diagnostic connector.
- Run the test as described in the tester manual.
- Repeat the manual bleeding procedure as outlined in Steps 1 through 6. Check and replenish the master cylinder fluid level if necessary.
After finishing, there should be no feeling of sponginess in the brake pedal. If there is, either there is still air in the line, in which case the process must be repeated, or there is a leak somewhere, which, of course, must be corrected before the vehicle is moved. After all repairs and service work are finished, road test the vehicle to verify proper brake system operation.