Uneven pad wear side to side, pulling under braking, a hot wheel after a run, and a smell of hot brakes.

Uneven wear between the inner and outer pad on the same caliper points at seized slide pins, because the caliper body can no longer move to balance the load.

Uneven wear between the two sides of an axle points at one caliper not applying or not releasing.

A wheel noticeably hotter than its opposite number after a drive is a dragging caliper, and the associated fuel consumption is measurable on a fleet.

Pulling to one side under braking is a caliper not applying, or a collapsed flexible hose acting as a one-way valve.

Check the hose before condemning the caliper - a hose that has failed internally gives identical symptoms and costs a fraction as much.

Repair kit where the bore is clean and the body is sound; replacement or exchange unit where the bore is pitted or the slides are corroded beyond recovery.

A repair kit typically contains the piston seal, dust boot, slide pin boots and sometimes the pins and a piston.

It is inexpensive and the right answer on a caliper that has simply aged, particularly on heavy or specialist vehicles where new units are costly.

Pitting in the bore is the disqualifier. A seal cannot conform to a pitted surface and will leak, and honing removes the surface hardening.

Remanufactured exchange calipers come with a warranty and a core charge, and once the labour of an uncertain repair is counted they are frequently cheaper.

Whichever route, do both sides of the axle if both have covered the same distance - the second will follow the first.

A high-temperature grease specified as compatible with the rubber boots and seals - not general-purpose or copper grease.

Ordinary greases attack rubber, swelling the boots so they trap grit rather than excluding it, which causes the exact seizure the grease was meant to prevent.

Silicone-based and specific synthetic brake greases are formulated for this and remain stable at brake temperatures.

Copper-based anti-seize belongs on pad backing plates and mounting hardware, not on slide pins where it will contaminate the boots.

Apply a thin film to the pin and inside the bore, then fit the boot without trapping air, so the pin can move freely through its full travel.

Check the pin moves smoothly by hand before reassembly. A pin that needs force is not clean enough.

Pushed straight in on ordinary calipers, but wound in with a tool on any caliper with an integrated parking brake - forcing one damages the mechanism.

A standard caliper piston is simply pressed back into its bore with a clamp, which displaces fluid back up to the reservoir.

A caliper with an integrated mechanical parking brake has a threaded piston that must be rotated as it retracts, usually clockwise, using a wind-back tool.

Forcing such a piston straight in destroys the internal adjuster and means a new caliper.

Electric parking brakes have to be put into service mode with a diagnostic tool before the piston will retract at all - and put back afterwards.

Watch the reservoir while retracting. Pushing fluid back can overflow it, and brake fluid strips paint immediately.

Whenever it is perished, swollen, chafed or the vehicle is old enough that its age is unknown - a failing hose mimics a seized caliper exactly.

Flexible hoses degrade from the inside as well as the outside, and internal delamination can create a flap that lets pressure in but not out.

The result is a brake that applies and does not release, which presents identically to a seized caliper and is much cheaper to fix.

External signs - cracking, bulging under pressure, chafing where the hose passes a bracket - are worth looking for at every service.

Hoses have a finite life independent of mileage, so age matters on vehicles that stand, such as trailers and seasonal plant.

Replacing hoses in axle pairs, with the caliper work, avoids a second bleed of the system a few months later.

A sliding caliper has pistons on one side and moves on pins so the outer pad is pulled into contact. A fixed caliper has pistons on both sides and does not move.

Sliding designs are simpler, cheaper and lighter, and dominate on ordinary vehicles.

Their weakness is the slide mechanism: if the pins seize, the outer pad stops working and the inner pad does all the braking.

A fixed caliper applies both pads directly, gives a firmer pedal and handles heat better, at higher cost and weight.

Fixed calipers have no slide pins to maintain, but they have twice as many piston seals to fail and they cannot be rebuilt as casually.

The practical difference at service time is that a sliding caliper needs its pins cleaned and greased at every pad change and a fixed one does not.

They are actuated by an air chamber through a mechanical amplifier rather than by hydraulic pressure, and their adjustment and inspection regime is its own discipline.

Air disc brakes are standard on much of the heavy commercial fleet and are increasingly replacing drums there.

An air chamber pushes a lever, which drives an internal mechanism that multiplies the force and applies the pads.

They incorporate automatic adjusters that take up pad wear, and a failed adjuster is a common fault that shows as excessive stroke at the chamber.

Chamber stroke is the standard inspection measurement and is checked against a limit, because excessive stroke means reduced braking force.

Guide pin and boot condition matter as much as on a light vehicle - more, because the loads and the service intervals are both larger.