Front brake pulsation and a right pull with sub-minimum discs
A 2014 sedan (front disc/rear drum, 128,000 km, no ABS) comes in with "the steering wheel shakes when I brake from highway speed and the car pulls right". You measure the front discs at 22.3 mm and 22.4 mm; the casting stamp shows MIN 24.0 mm. Disc runout with a dial gauge is 0.10 mm on the left and 0.04 mm on the right (limit 0.05 mm). Pad friction material is 3 mm on the left inner pad and 8 mm elsewhere. The left caliper guide pins are dry and one is seized; the right side slides freely. Tyre pressures are 210 kPa all round as specified. Wheel bearings show no play. The customer supplies two new aftermarket discs and a used caliper from another car and wants them fitted to save money.
1. Which measurements decide whether the discs are replaced or machined, and how should they be taken?
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Thickness decides replacement, not runout. Both front discs measure 22.3 mm and 22.4 mm against a minimum of 24.0 mm, so they are already below the wear limit and must be replaced. Machining would remove more material, leave them thinner than the manufacturer allows, and increase the risk of overheating, warping and cracking. Runout only identifies which side is causing the pulsation: the left at 0.10 mm exceeds the 0.05 mm limit, while the right at 0.04 mm is within it.
Measure thickness with a micrometre at about 12 points around each disc, not with a caliper, and record the minimum. Before fitting new discs, clean the hub face with a wire brush or abrasive so rust or debris does not create runout. Check disc runout after installation with the wheel nuts evenly torqued.
2. Explain the pulsation and the right pull using the measurements, and say why new discs alone will not solve both complaints.
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The pulsation is caused by disc thickness variation and runout. The left disc's 0.10 mm runout pushes the pads apart and lets them back in once per revolution, producing a pressure pulse felt through the steering wheel and pedal at highway speed. The right disc is within runout, so it is not the main source.
The right pull under braking is a separate fault. The left caliper's seized guide pin prevents the caliper from floating and applying full force, so the right side does more of the braking and the car pulls right. The 3 mm left inner pad supports uneven application and drag. Fitting new discs will cure the pulsation but leave the pull, because the caliper fault remains. Both faults must be corrected, and the pads should be replaced as an axle set.
3. How do you respond to the request to fit a used caliper from another car, and what checks follow the repair?
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Decline to fit the used caliper as a false economy. It may carry the same corrosion or a damaged piston seal, may have a different piston diameter, and its history cannot be verified. Offer a new or professionally rebuilt caliper with new seals, new guide pins and boots, and new pads on both sides. Explain that you cannot warranty a safety-critical braking repair using customer-supplied used parts, and record that advice on the job card.
After the repair: clean and lubricate the slides with brake grease, check the flexible hose for cracks or swelling, bleed in the correct sequence, and confirm a firm pedal with no creep. Road-test at low speed first, then at highway speed with light braking to confirm no pulsation or pull. Re-check wheel nut torque and inspect for fluid leaks before handover.