Brake Pulsation, Disc Runout and a Long Pedal After an ABS Unit Replacement
A 2018 1.6 petrol hatchback, 62,000 miles, has pedal and steering-wheel pulsation when braking from 50 to 70 mph, worst when the brakes are warm. Front discs and pads were replaced 9,000 miles ago at another garage using a budget brand. The wheels were removed and refitted with an impact wrench, with no torque wrench used. Your measurements: front disc runout 0.09 mm at the outer edge against a manufacturer limit of 0.05 mm; disc thickness variation 0.03 mm; hub flange runout 0.02 mm against a limit of 0.03 mm; pad friction material 7 mm both sides; slide pins dry with slight corrosion; rear discs 9.5 mm against a 9.0 mm minimum. The customer also says the pedal has felt long since an ABS hydraulic unit was replaced two months earlier. That system was bled at the calipers only by pedal pumping, with no diagnostic equipment used. The ABS warning lamp is off.
1. Diagnose the pulsation. Explain what runout, thickness variation and hub flange runout tell you, how uneven pad deposits form, and why the wheel fitting method matters.
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The pulsation is almost certainly front brake torque variation, not an ABS fault, because the ABS lamp is off and the symptom is speed-related and worse when hot.
The measurements tell a sequence. Hub flange runout of 0.02 mm is within the 0.03 mm limit, so the hub is acceptable. Disc runout of 0.09 mm is nearly double the 0.05 mm limit, so the disc is not running true. Thickness variation of 0.03 mm means the disc is also uneven in thickness around its circumference. Either condition alone can cause pulsation, and both together are conclusive.
The mechanism is usually uneven pad transfer. When a disc does not run true, or the wheel is not seated evenly, the pad contacts the high spots harder, deposits friction material there, and that patch grabs more, which deposits more. The result is a disc that varies in friction and often in thickness, felt as a shimmy through the wheel and a pulsing pedal. Heat accelerates it, which matches the customer's description.
Impact-wrench tightening matters because uneven or over-tightened wheel nuts distort the disc and the hub face, and they can also damage the studs and the wheel seat. That distortion becomes permanent once the brakes have been hot. The correct method is a torque wrench in a star sequence in stages to the manufacturer's figure. The dry slide pins are a secondary fault that will cause uneven pad wear and drag.
2. Specify the repair. Would you machine the discs or replace them, and what checks and procedures would you insist on before and after fitting?
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I would replace the front discs and pads rather than machine them. The discs are budget items with 9,000 miles and a measured runout of 0.09 mm plus 0.03 mm thickness variation; machining removes material from an already marginal disc, leaves it thinner and closer to its minimum, and will not fix the underlying cause if the hub face or the wheel seating is the problem. Replacement also gives a known starting point for the warranty.
Procedure: measure hub flange runout again after cleaning the hub face with a wire brush or abrasive disc and checking for burrs, rust or trapped debris; a 0.02 mm reading is only meaningful on a clean face. Check the caliper pistons for free movement and corrosion, clean and lubricate the slide pins with the specified grease, and check the caliper carrier and abutment clips. Fit the new discs, check runout on the vehicle with a dial gauge, then fit pads with the correct shims. Refit the wheels by hand, then torque in a star sequence in stages to the manufacturer's figure with a calibrated wrench. Check the rear discs: 9.5 mm against a 9.0 mm minimum leaves little life, so I would advise replacement, but they are not the cause of the pulsation.
After the repair, bed the pads in per the manufacturer's procedure: a series of moderate stops from moderate speed with cooling intervals, not a series of hard stops from 70 mph. Road test and record the result.
3. Address the long pedal. Explain why pedal bleeding alone is inadequate after an ABS hydraulic unit replacement, and set out the correct process and final quality checks.
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A long pedal after a hydraulic unit replacement indicates air trapped in the unit, most likely in the valve block or the pump chamber. Pedal bleeding at the calipers pushes fluid through the open circuits, but it does not cycle the solenoid valves, so air sitting in the closed or dormant galleries stays there. The pedal then feels long because the first part of the travel is compressing air, not moving fluid.
Correct process: use the scan tool's bleed or actuator-test function to cycle the pump and valves in the sequence the manufacturer specifies, keeping the reservoir topped up throughout and never letting it run dry. Bleed at the calipers in the specified order, using the correct fluid specification, with a pressure or vacuum bleeder, and check the fluid's water content and boiling point if it has not been changed in two years. Use new fluid from a sealed container, and dispose of the old fluid properly. Some units need a specific pedal-hold routine during the actuator test; follow the data, not habit.
Quality checks: firm pedal with no sponginess or sink under sustained pressure, no leaks at the unions, correct fluid level, ABS lamp off with no stored codes after a road test, and an ABS activation test at low speed on a safe, dry surface. Record the fluid used, the bleed sequence, the scan-tool results and the road-test outcome on the job card. Before the car leaves, re-check the wheel nut torque and tell the customer about the pad bedding-in period.