Pulsating Brakes with Rotors Below Minimum Thickness
A 2016 sedan, 78,000 km, comes in with a pulsating brake pedal and steering wheel shimmy when braking from 80 km/h, plus a slight pull to the right. Front pads were replaced 9,000 km ago, but the rotors were only measured, not machined. Measurements: left front rotor thickness 24.6 mm, right 25.1 mm, minimum thickness 26.0 mm; left front lateral runout 0.12 mm (maximum 0.05); hub runout 0.03 mm; right front caliper slide pins free; left front lower slide pin seized, inner pad 2.0 mm, outer pad 7.5 mm. Rear brakes are drum type, shoes 4 mm, drum diameter within limits. Brake fluid moisture is 3.2% and it was last changed four years ago. The owner declines both front rotors and asks the workshop to just skim them and change the pads because a relative can do it cheaper.
1. Decide whether skimming the front rotors is acceptable and explain how the measurements support your decision.
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Skimming is not acceptable. The minimum thickness is 26.0 mm; both rotors are already below it (24.6 and 25.1 mm), so machining would make them thinner, reduce heat capacity and increase the risk of cracking or fade. Even if thickness were adequate, left lateral runout 0.12 mm exceeds 0.05 mm; hub runout 0.03 mm is within limits, so the runout is in the rotor, not the hub. Pulsation is caused by thickness variation/runout, and the seized left slide pin explains the inner pad at 2.0 mm versus outer 7.5 mm and the pull. A pad slap or skim would leave uneven clamping and the pulsation would return. Correct repair: replace both front rotors and pads as an axle set, free or replace the seized slide pin and boot, clean and lubricate with high-temperature brake grease, and check caliper piston retraction.
2. List the repair priorities and justify the order, including what to do about the brake fluid and rear brakes.
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Safety-critical items come first: front rotors, pads and the sticking caliper, because braking performance and vehicle control are affected. Brake fluid at 3.2% moisture is well above the usual 2% replacement threshold and has been in service four years; moisture lowers boiling point and promotes corrosion, so flush with the specified fluid. Rear shoes at 4 mm may still be serviceable depending on the limit, but measure drum diameter and shoe thickness against data rather than guessing. Road-test at low speed first, then carry out a controlled higher-speed brake test, and recheck pedal feel and wheel temperatures. This order prevents a comeback: fixing pulsation without the caliper would quickly ruin the new rotors.
3. How should the workshop respond to the customer's refusal, and what should be documented?
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Explain the measurements in plain language, show the rotor thickness against the minimum and the runout reading, and state that the vehicle is below the manufacturer's safe limit. Do not release a vehicle as repaired when it is not. If the customer insists on a cheaper partial repair, record the refusal and the safety advice on the job card and invoice, and have the customer sign. A workshop must not carry out work that leaves the vehicle unsafe. Offer a quality aftermarket option if cost is the issue, but never machine below minimum. If the customer takes the vehicle away, note that the brakes may pull or fade and advise against towing. This protects both the customer and the workshop's liability.