Brake pulsation, wheel-bearing play and a minimum safe repair
A 2019 rear-wheel-drive light commercial van (3.5 t) comes in after the driver reports a pulsating brake pedal, a pull to the right when braking from 70 km/h, and a metallic groan from the front left. Inspection: front left outer pad 3 mm, inner pad 8 mm; right outer 7 mm, inner 7 mm; front left disc thickness 26.8 mm (minimum 28.0 mm), runout 0.11 mm (max 0.05 mm); front right disc 29.1 mm, runout 0.03 mm; left front wheel bearing axial play 0.12 mm (max 0.05 mm); caliper guide pins free; left flexible hose has no external cracks; brake fluid moisture 3.2% (max 3%); tyres matched, pressures correct. The van is used daily for deliveries and is booked for a 400 km trip tomorrow. The customer authorises only the minimum needed for safety. The workshop must decide what can be reused, what must be replaced, and whether the van can be released.
1. Evaluate the measurements against the limits and identify the most likely causes of the pulsation, pull and groan. Explain why pad thickness alone is misleading.
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The front left disc is below the minimum thickness: 26.8 mm against 28.0 mm, so it has only 26.8/28.0 of the minimum material and must not be resurfaced or reused. Its runout of 0.11 mm is more than double the 0.05 mm limit and would produce pedal pulsation. The left wheel-bearing axial play of 0.12 mm exceeds 0.05 mm and can add disc runout, pad knock-back and uneven pad wear. The right disc is above minimum at 29.1 mm with acceptable runout, but brake parts should generally be replaced as an axle set to keep balanced braking.
The left pad taper (3 mm outer versus 8 mm inner) points to a caliper piston that is not releasing or applying evenly, or to an internally collapsed hose; free guide pins make a slider fault less likely. The metallic groan from the left is consistent with metal-to-metal contact or a dragging caliper. The pull to the right may be because the left brake is dragging and fading, or because the right brake is applying more effectively. Brake fluid moisture at 3.2% exceeds the 3% limit and should be flushed.
Pad thickness alone is misleading because a pad can be above the wear limit while the disc is below minimum, the runout is excessive, the bearing is loose, or the caliper is sticking. Safety is governed by the whole assembly, not one pad reading.
2. Decide what can be reused and what must be replaced to release the van for the 400 km trip. Explain why the customer's 'minimum' cannot include reusing the left disc.
Read the worked answer
The van should not be released as it stands. The minimum safe repair is: replace both front discs and the front pad set as an axle set; replace the left front wheel bearing/hub; inspect the left caliper piston and hose and replace or remanufacture the caliper if the piston is sticking or the hose is suspect; flush the brake fluid to below 3% moisture; then road-test and recheck for pulsation, pull and excessive hub temperature.
The left disc cannot be reused because it is already 1.2 mm below the minimum thickness and has excessive runout. Machining it would make it thinner and reduce heat capacity further. Replacing only the left disc while leaving the right disc and mismatched pads is poor practice and can create uneven braking. If the right disc is reused against new left-side friction, the axle set will not be balanced. If correct parts are not available before the trip, the workshop should refuse release and offer a loan vehicle or delay the trip; a 400 km delivery run with a below-minimum disc and a loose wheel bearing is an unacceptable liability.