Front Brake Drag After a Pad Replacement
A 2019 150 cc scooter with a single front disc and no ABS returns 60 km after you replaced the front pads. The owner says the front wheel is hot after a short ride and the machine is hard to push backwards. The brake lever has almost no free play. You find the master-cylinder reservoir filled to the top with DOT 4, new pads fitted without the anti-rattle spring, caliper guide pins coated with ordinary chassis grease, and a fork seal that has been weeping onto the left pad. The front wheel binds when spun. The disc measures 3.8 mm, with a stamped minimum thickness of 4.0 mm; runout with a dial gauge is 0.25 mm; the service limit in the manual is 0.20 mm.
1. Identify the causes of the drag and rank them by safety significance. Explain what each does.
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Ranked by safety significance: (1) The disc is below minimum thickness (3.8 mm against 4.0 mm) and runout is 0.25 mm against a 0.20 mm limit. A thin, warped disc overheats, can crack and gives poor braking. (2) The left pad is contaminated with fork oil; contaminated friction material cannot be restored and may pull or fade. Chassis grease on the guide pins can swell the rubber boots and attract dirt, so the caliper does not slide and the pads drag. (3) The reservoir is overfilled; as the fluid heats it expands with nowhere to return, so pressure builds and the brake drags, which explains the hot wheel and the near-zero lever free play. (4) The anti-rattle spring is missing, so the pads can sit cocked and fail to retract. The fork seal leak is both a cause of contamination and a separate fault.
2. State exactly what you would replace, clean or adjust, and what you would not do. Include the measurement decisions.
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I would replace the disc-it is below minimum and over the runout limit-and replace the pads as a set because they are contaminated; I would not sand or clean them. I would replace the leaking fork seal and clean the fork oil from the caliper area. I would remove the caliper pistons, clean the bore and pistons with brake cleaner, fit new seals if swollen or damaged, and lubricate the guide pins with brake-caliper high-temperature grease, not chassis grease. I would fit the anti-rattle spring, flush the fluid with fresh DOT 4, fill to the correct level-not to the top-and set lever free play. I would check the master-cylinder return port is clear, check the disc mounting bolts and torque to spec, then road test. I would not skim the disc or reuse the contaminated pads.
3. The customer disputes the cost because the bike braked fine when it left. How would you explain the failure and the need for the disc replacement using the measurements?
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I would show the customer the stamped 4.0 mm minimum and the 3.8 mm measurement, and the dial-gauge reading of 0.25 mm against 0.20 mm. Those are objective condemning limits: a disc that thin has less heat capacity, is more likely to warp and can crack, so replacing it is a safety decision, not an upsell. I would explain that the brake can feel normal when cold; the overfilled reservoir and the dragging caliper then heat the fluid, and the missing spring and wrong grease keep the pads applied. The fork leak contaminated the pad, and oil-soaked friction material cannot be restored. If my work caused the overfill, missing spring or wrong grease, I would own that part and explain what is being done. I would not return the bike with a below-minimum disc; the customer can have the old parts for inspection.