One wheel drag on disc brakes.
Cause of One Wheel Drag
There are several different possible causes for one wheel drag. The cause of a one-wheel drag on a disc brake can be either mechanical or hydraulic. 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)
One Wheel Drag Solution
Applying a systematic approach to the diagnosis of a one-wheel drag condition will determine the cause. The diagnosis’s objective is to 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 the dragging wheel. Check the drag condition if the drag releases; go to step 5. If the drag is still present, go to the next step.
NOTE: If it is impossible to loosen the brake hose inlet fitting go to the next fitting upstream. You can usually find a fitting higher than the hose you can loosen without damaging 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 applying but not allowing full release (see Figure 54.1). If the drag does not release, go to the next step.
One Wheel Drag - Steps 4-7
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 is releasing after step 2, tighten the 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 the 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 another component). Check drag. If released, the restriction is in the ABS modulator, and modulator replacement is required.
7. If drag is releasing after step 5 – tighten brake line fitting at the ABS 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 the combination valve goes to step 8 if the master cylinder on the FWD vehicle goes to step 9.
One Wheel Drag - Steps 8-10
8. Combination valve on RWD vehicles – If loosening the inlet on the combination valve released, the drag goes to step 9. If wheel drag is still present, loosen the outlet line on the combination valve that supplies the dragging wheel. If the wheel releases, a combination valve is the source of restriction and will need replacing.
9. Tighten all fittings and identical drag conditions. 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 the master cylinder. This could include stoplight switch adjustment, pushrod adjustment, partial assist condition, or pedal height adjustment. If the 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 the wheel frees up master cylinder is the source of drag and will require replacement.
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