It is cast or stamped on the disc itself, usually on the hub face or the outer edge, and it is the point at which the disc must be scrapped.

The figure exists because a thin disc has less mass to absorb heat and less material to resist cracking.

It is a scrap thickness, not a machining thickness. A disc being machined must finish above the minimum with enough left for the next set of pads.

Measure with a micrometer at several points around the disc, not with a vernier at the edge, because the worn area sits inboard of the outer lip.

Measuring at several points also reveals thickness variation, which is the measurement that explains a pulsing pedal.

On many modern light vehicles the new thickness is close enough to the minimum that machining is not viable at all.

Almost never. It is disc thickness variation, produced by excessive run-out or an uneven pad transfer layer.

A genuinely warped disc - permanently distorted by heat - does happen, but it is far rarer than the diagnosis suggests.

What usually happens is that the disc runs slightly out of true, so one region passes the pads more firmly, wears differently, and becomes measurably thinner than the rest.

As the caliper piston moves in and out over that variation, the pedal pulses.

The other cause is an uneven transfer layer from poor bedding-in or from holding the brake while hot, which builds up material in patches rather than removing it.

Both are prevented at fitting: clean the hub face to bare metal, check run-out with a dial gauge against the manufacturer's limit, and bed the pads in properly.

For most vehicles, no. They help clear gas and debris from the pad face, and drilling removes material and adds stress raisers.

The original purpose was to vent gases released by older friction materials, which modern compounds produce far less of.

Slots do usefully keep the pad face clean and give a slightly more consistent bite in the wet, at the cost of faster pad wear.

Cross-drilling reduces the disc's mass, which reduces its heat capacity, and each hole is a place cracks can start under thermal stress.

For heavy vehicles, towing, and any sustained-braking duty, a plain vented disc with good vane design outperforms a drilled one.

Where discs are cross-drilled by the manufacturer, they are usually cast with the holes rather than drilled afterwards, which is a different and much better proposition.

Vented wherever the duty generates real heat - which is most front axles and any loaded or towing application. Solid suits light rear-axle duty.

A vented disc has two friction faces separated by vanes that pump air outwards as the disc turns, and it sheds heat far faster than a solid disc of equal weight.

Front brakes do most of the work on most vehicles because weight transfers forward under braking, so they are vented almost universally.

Rear axles on light vehicles often use solid discs perfectly adequately.

Vane design varies - straight, curved and pillar patterns behave differently - and curved-vane discs are handed, so left and right are not interchangeable.

Fitting a handed disc on the wrong side reverses the pumping action and loses most of the cooling, which is an easy and invisible mistake.

Clean the hub face to bare metal, check run-out with a dial gauge, remove the protective coating from the friction faces, and torque the wheel properly.

Corrosion or debris on the hub face is the leading cause of run-out. A speck of rust under the disc tilts the whole face.

Run-out should be measured with a dial gauge against the manufacturer's limit, not assumed - it takes a couple of minutes and prevents a comeback.

New discs carry a protective coating that must be removed from the friction faces with the recommended cleaner, or the first stops will glaze the surface.

Wheel nuts must be torqued evenly to specification with a torque wrench. Over-tightening or uneven tightening distorts the disc and produces run-out from the first kilometre.

Bed the pads in afterwards. A new disc and old pads, or new pads not bedded, both produce the pulsing that the new parts were meant to cure.

Light rust after standing wipes off in the first few stops. Deep pitting, or rust in the area the pad does not sweep, is a different matter.

Cast iron discs rust within hours in damp air, and a vehicle left over a weekend will have visibly rusty discs on Monday. That is normal.

Vehicles that stand for months - trailers, seasonal plant, reserve fleet - develop pitting that does not clean off, and the pitted surface eats the new pads.

Rust build-up on the unswept edge and in the hub area lifts the disc off the hub face and creates run-out, which is why that area is cleaned at every fitting.

Corrosion between the disc and the hub is also what makes a disc almost impossible to remove years later.

Coated and painted discs resist the cosmetic rust on the non-friction areas and are worth specifying for vehicles that stand.

A disc with a small drum brake built into its centre section, used as the parking brake on vehicles with rear disc brakes.

Rear disc brakes make a mechanical parking brake awkward, because a caliper is designed for hydraulic actuation.

One solution integrates a small drum into the disc's hat, with its own shoes operated by cable, entirely separate from the service brake.

The other integrates a mechanical mechanism into the caliper piston, which is why some calipers must be wound back rather than pressed.

A drum-in-hat parking brake has its own shoes, springs and adjuster that need inspecting, and they are frequently forgotten because they wear very slowly.

When a rear disc is replaced, those internals become accessible - which is the moment to check them rather than the next time the parking brake fails a test.