Below-Minimum Discs, a Customer in a Hurry and a Fluid Refusal
A 2018 seven-seat MPV with 68,000 km comes in with the steering wheel shaking when the brakes are applied from about 100 km/h, worse after a long downhill descent. On the hoist: front pads 3 mm; front discs measure 24.4 mm with a micrometer at eight points, and the minimum thickness stamped on the disc hat is 26.0 mm; disc lateral runout is 0.09 mm; hub-face runout is 0.04 mm with the disc removed; disc thickness variation is 0.025 mm. Rear drums and shoes are within specification. The brake fluid is dark and the moisture tester reads above 3 %, with no record of a fluid change in the service history. The customer is selling the car next month and asks you to just machine the discs, fit cheap pads and leave the fluid alone, saying it is a waste of money. The workshop's waste contractor collects used oil only; there is no contract for brake fluid.
1. Using the measurements, decide whether the discs can be machined and explain what else must be corrected so the new discs do not develop the same fault.
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Machining is not an option The disc is 24.4 mm against a stamped minimum of 26.0 mm - already 1.6 mm below the limit before any cut. Machining removes material from both faces, and the disc would end up thinner still, with less heat capacity, a greater chance of cracking and pad knock-back as the piston over-extends. Both front discs and the pads must be replaced.
Why new discs alone can repeat the fault The judder is caused by thickness variation (0.025 mm here is enough to be felt) and runout of 0.09 mm, well over a typical 0.05 mm limit. With the disc off, the hub face still shows 0.04 mm, so part of that runout is in the hub, not the disc. If the hub is not corrected, the new disc will be forced to run out and will wear into the same DTV pattern. So: clean the hub face of rust, scale and burrs; measure hub runout again; check wheel bearing play; check for a trapped shim or a damaged stud; and torque the wheels with a torque wrench in a star pattern rather than with a rattle gun, which is a classic cause of DTV.
After fitting, measure runout on the assembled disc with the wheel nuts and washers at the specified torque, confirm under 0.05 mm, bed the pads in, and check pad thickness, caliper slide pins and hose condition.
2. The customer refuses the brake fluid change. What do you do, what has to be written down, and why does 3 % moisture matter?
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Why it matters DOT 4 fluid absorbs moisture, roughly 1–2 % per year. Dry boiling point is around 230 °C; at 3 % water it falls to roughly 150–160 °C. The customer's own complaint is hard braking on a long descent - exactly the condition that boils wet fluid. Vapour in the caliper is compressible, so the next application takes the pedal near the floor. Moisture also corrodes calipers, lines and the ABS modulator from the inside, which is far more expensive than a fluid change.
What to do Explain this once, clearly, with the moisture tester reading shown to the customer. Do not simply agree and stay silent, and do not perform the brake job in a way that implies the system is safe.
What to write
- On the job card: fluid moisture above 3 %, advised replacement, customer declined, date, time and the adviser's name.
- On the invoice and the vehicle inspection sheet: fluid listed as a defect, marked as declined by the customer.
- Ask the customer to acknowledge in writing, and keep the copy. If the car is being sold, say plainly that the new owner inherits a safety defect. Release the car only after the brakes you did repair are tested; the refusal concerns the fluid, not the discs.
Cheap pads on below-minimum discs is not a compromise worth offering: it will judder again and the customer will blame you.
3. How should the old brake fluid, contaminated rags, brake cleaner and the worn discs be handled, given the workshop has no brake-fluid contract?
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Do not improvise Brake fluid is hazardous: it is harmful if swallowed, damages paint and is not acceptable to the drain, the ground or the general waste bin. Since the contractor only takes used oil, the correct response is to stop and fix the gap before the next brake job: set up a labelled, closed container for waste brake fluid, store it in the designated locked waste area on a spill tray, and arrange a licensed contractor with a manifest or consignment note. Until then, fluid drained from a vehicle stays in that container and is not poured into the waste oil tank or mixed with it.
Other streams
- Rags and absorbent granules contaminated with fluid or solvent go into a closed, labelled container as contaminated waste, not the general bin.
- Brake cleaner is used with ventilation, away from sparks and hot work, and the rags are treated as solvent waste. Never let it run to the drain; the workshop's oil/water separator is not a disposal route.
- Brake dust is not blown out with air; use a damp method or a HEPA vacuum, and bag the filter.
- Worn discs, drums and pads are scrap metal: put them in the metal recycling bin, not general waste. Keep a spill kit with granules and a scoop near the brake bay, and record the quantities leaving the workshop so the waste trail can be shown on inspection.