Problem: Both front wheels experiencing wheel drag
Cause: Residual pressure causing brakes to slightly apply after brakes reach operating temperature. The most common cause for this condition is plugged or covered vent ports in master cylinder.
Solution: The cause of the residual pressure must be identified. When presented with a two wheel drag complaint the diagnostic process starts under the hood not at the wheel. Both wheels having the same symptom rules out the calipers and brake hoses as likely causes. The cause will almost always be something common to both wheels (See Figure 55.1).
NOTE: Before proceeding with diagnosis check system for contamination.
- Stay close to the shop
- Take a line wrench with you in case the wheel drag increases to a point where the vehicle isn’t drivable
- Do not “hammer” the brakes during the test drive. make numerous stops as if in city driving until you either feel the drag or know the brakes are at operating temperature.
Follow these steps to diagnose the cause of a two wheel drag complaint:
1. Once the brakes are at operating temperature or the drag has been duplicated rack the vehicle and leave it in neutral.
2. Check all four wheels for drag. If necessary measure drag.
3. Drag on front brakes (or front and rear on 4 wheel disc brake equipped vehicles) – start by loosening the master cylinder away from the vacuum booster by ¼” . Check effect on drag condition. If drag is still present go to next step. If drag is released go to step eight.
4. Drag still present with master away from booster – Loosen the brake line(s) supplying the dragging brakes. Check effect on drag. If drag is released there is an internal problem in master cylinder. Either the vent port(s) are plugged or the piston(s) are binding in the bore. Master cylinder replacement is required. If drag is not released go to next step.
5. Drag is not released after brake lines at master cylinder are loosened. Locate next component downstream from master cylinder. Trace brake lines from front wheels to that component. Loosen outlet lines of component that supply the dragging brakes. If drag is released component is source of restriction and requires replacement. If drag is not released go to next step.
6. Drag is not released after loosening brake lines at outlet of component between master cylinder and dragging brakes. Trace lines to dragging brakes. Is there another component between component tested in step 5 and dragging brakes? If yes, repeat step 5 on that component. If no, go to next step.
7. Wheel drag still present on both wheels after performing steps 3 through 6. Open bleeder screws on both front calipers and check effect on drag condition. If either wheel frees up, the brake hose on that wheel is acting as a one-way check valve. If either or both of the wheels fail to release the problem is a mechanical problem with both front calipers. Inspect and service as required.
8. Wheels released after master cylinder was moved away from vacuum booster. Problem is in front of master cylinder. Something is not allowing pistons in master cylinder to return to a full released position resulting in the cup seals covering the vent ports. The list of possible causes varies from vehicle to vehicle based on system configuration. Here is a list of possible causes (see Figure 55.2):
- Stop light switch adjustment.
- Binding pedal linkage – check for free movement. If necessary disconnect booster pushrod.
- Pedal height adjustment – some import vehicles have an adjustable link between brake pedal and booster.
- Vacuum booster applying partial assist – air and vacuum valves inside booster may not be returning to their proper position when brake pedal is released. Pull one-way check valve out of booster with drag present to check for this condition.
- Adjustable pushrod on booster is too long.
NOTE: Most 2 wheel drag problems will be caused by either a problem in front of the master cylinder as described above or by the master cylinder itself. Two wheel drag problems are rarely caused by problems below the master cylinder.