Problem: One wheel drag on disc brake
Cause: There are a number of different possible causes for one wheel drag. The cause of a one wheel drag on a disc brake can be either mechanical or hydraulic in nature. The list of possible causes will include:
- Caliper slides
- Caliper piston
- Check valve brake hose
- Restriction in ABS modulator (if equipped)
- Plugged or covered vent port (FWD only)
Solution: Applying a systematic approach to the diagnosis of a one wheel drag condition will determine the cause. The objective of the diagnosis is to not only determine if the source is mechanical or hydraulic but to determine the actual cause.
To diagnose a one wheel drag on a disc brake follow the steps below:
1. Duplicate the problem. Confirm the problem before continuing. If necessary measure wheel drag.
2. Loosen the brake line fitting at the INLET end of the brake hose supplying dragging wheel. Check the drag condition. If the drag releases go to step 5. If the drag is still present go to next step.
NOTE: If it is not possible to loosen the brake hose inlet fitting go to the next fitting up stream. You can usually find a fitting higher than the hose that can be loosened without damage to the brake line.
3. Tighten brake line fitting. Loosen bleeder screw on dragging caliper. If the drag releases the brake hose is the source of the problem and requires replacement. It is acting as a one-way check valve. Fluid is allowed to the caliper on apply but not allowing full release (see Figure 54.1) If the drag does not release go to next step.
4. If drag is still present after steps 1 to 3 the problem is mechanical in nature. The caliper piston or caliper slides are causing the drag. An inspection of the caliper should yield the source.
5. If drag released after step 2 tighten fitting and duplicate the problem. Locate the next component higher than the brake hose. This will most likely be the ABS modulator. With the wheel drag present, loosen the inlet brake line supplying the dragging wheel. Check effect on drag. If drag does not release go to next step. If drag does release go to step 7.
6. Drag does not release after step 5 – loosen outlet brake line fitting at ABS modulator (or other component). Check drag. If released, restriction is in ABS modulator and modulator replacement is required.
7. If drag released after step 5 – tighten brake line fitting atABS modulator. Duplicate drag condition. Loosen inlet brake line fitting on next component upstream. This will usually be the combination valve on RWD vehicles and the master cylinder on FWD vehicles (See Figure 54.2). If combination valve go to step 8, if master cylinder on FWD vehicle go to step 9.
8. Combination valve on RWD vehicles – If loosening the inlet on the combination valve released the drag go to step 9. If wheel drag is still present loosen outlet line on combination valve that supplies dragging wheel. If wheel releases combination valve is source of restriction and will have to be replaced.
9. Tighten all fittings and duplicate drag condition. With drag condition present loosen master cylinder from power assist unit by at least ¼”. Check drag. If drag releases problem is in front of master cylinder. This could include stoplight switch adjustment, pushrod adjustment, partial assist condition or pedal height adjustment. If drag condition is still present go to step 10.
10. With drag condition present, loosen brake line fitting at master cylinder outlet supplying dragging brake. If wheel frees up master cylinder is source of drag and will require replacement.