Pulsating Brakes and a Pull to the Right
A light commercial van with front discs and rear drums comes in with a pulsating brake pedal, a steering wheel that shakes when braking from 80 km/h, and a pull to the right whenever the brakes are applied. New front discs and pads were fitted 18 months ago.
Micrometer readings at eight points, 45° apart, 10 mm in from the edge: left disc 23.6 mm, right disc 24.1 mm. Lateral runout with bearing play checked and the disc nut torqued: left 0.11 mm, right 0.04 mm. Disc data: new 26.0 mm, minimum thickness 24.0 mm, maximum runout 0.05 mm. Pad friction material: left inner 2 mm, left outer 7 mm, right inner and outer 6 mm. The left caliper slide pins are dry and corroded; the right side moves freely. The customer asks for the discs to be machined rather than replaced.
1. Decide for each front disc whether to machine or replace, showing the arithmetic against the 24.0 mm minimum.
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Left disc - scrap, not machinable. It measures 23.6 mm against a minimum of 24.0 mm, so it is already 0.4 mm below the discard thickness. It is also the disc with 0.11 mm runout against a 0.05 mm limit, so it is the source of the pulsation. There is nothing to machine: it is finished.
Right disc - replace, do not machine. It measures 24.1 mm, leaving only 0.1 mm of margin above the minimum. A resurface removes roughly 0.1–0.2 mm from each face, i.e. 0.2–0.4 mm from the thickness, so any cut takes it below the limit. A disc under minimum thickness cannot absorb the heat of a stop: it warps again quickly, can crack, and gives a long pedal as the pads are knocked back. It is also an unsafe and non-compliant repair.
Both discs are replaced as a pair. Different disc thicknesses left to right produce unequal braking force and a pull, and the pair must be within about 0.1 mm of each other. New pads go on with the new discs; the old pads are bedded to the old, tapered surfaces.
2. Account for the left inner pad being 2 mm while the left outer pad is 7 mm, and explain why the van pulls right only when the brakes are applied.
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The 5 mm difference across the same caliper is the signature of a caliper that cannot slide. On this sliding-caliper design the piston pushes the inner pad directly; the reaction then has to slide the caliper body on its pins so the outer pad is pulled into the disc. With the left pins dry and corroded, the body is stuck, so the inner pad does nearly all the work and wears from 7 mm down to 2 mm, while the outer pad barely touches the disc.
The left wheel is therefore braking on one pad only, and with reduced force. Under braking, the right wheel produces normal torque and the left produces much less, so the resulting yaw moment drags the van towards the right - the side with the working brake. Without braking there is no pull, because the drag is small and both wheels are free.
Note the difference in behaviour: a caliper that is stuck on (dragging) gives a pull to its own side and heat at that wheel even when coasting. This one is stuck off, so the fault only shows under braking. The seized pin also explains the 2 mm pad - metal-to-metal is close, which is consistent with the left disc being worn below minimum thickness before its time.
3. The customer insists the discs be machined because they are only 18 months old. Write your technical reply, including what you would check to explain why an 18-month-old disc is already below minimum thickness.
Read the worked answer
Reply to the customer: machining removes metal; it cannot put any back. The left disc is already 0.4 mm under the manufacturer's minimum and the right has only 0.1 mm to spare, so there is no safe cut to take. A disc machined below minimum will overheat, warp and pulse again within weeks, and may crack - so machining is not a saving, it is paying twice and driving on a brake that will not do its job. Both discs are being replaced as an axle set, with new pads and overhauled calipers.
Why an 18-month-old disc is already worn out - checks to make:
- The left caliper slide pins, already found seized: the inner pad has been dragging, wearing pad and disc continuously rather than only when braking.
- Sideways runout re-measured properly, with the hub face cleaned of rust and scale and the wheel nuts torqued in sequence - dirt under the disc is a common false reading.
- Pad quality and fit: pads seized in the carrier also hold the disc in light contact.
- Disc quality: check the casting for a minimum-thickness marking; some aftermarket discs start close to minimum.
- Rear brakes: drums out of round also cause pedal pulsation, and should be measured and reported so the customer does not come back with the same complaint.