The self-servo effect gives high braking force for low input effort, and a drum encloses a large, protected handbrake mechanism.

The rotating drum drags the leading shoe into itself, multiplying the force applied by the wheel cylinder or air chamber.

That makes drums efficient on trailers and heavy vehicles, where available actuation force is limited and braking demand is high.

A drum also encloses its friction surfaces, which protects them from water, road salt and grit - an advantage on trailers that stand outside for long periods.

The parking brake integrates naturally into a drum, where a disc brake needs either a separate drum-in-hat arrangement or an electric caliper.

The trade is heat: a drum encloses its heat as well as its friction, so it fades sooner than a disc under sustained braking.

It is the drum's rotation dragging the leading shoe harder into contact, multiplying the applied force - and it is why drum braking is sensitive to heat.

The leading shoe is wedged into the drum by the drum's own movement, so a modest actuation force produces a large braking force.

Because the multiplication depends on the friction coefficient, anything that lowers friction lowers it twice: once directly and once through the lost servo effect.

That is why drum brake performance falls off more sharply with heat, and why fade arrives more suddenly than on a disc.

The effect is also directional. Braking in reverse loads the shoes differently, which is normal rather than a fault.

Practically: drums suit repeated moderate braking and are less suited to sustained heavy braking such as long descents, where engine or auxiliary braking should carry the load.

The spring and adjuster kit, every time - springs lose tension with heat cycling and a seized adjuster is the commonest drum brake fault.

Return springs pull the shoes off the drum. Weak springs leave the shoes dragging, which overheats the drum and wears the lining rapidly.

Hold-down springs and pins locate the shoes against the backplate and are cheap enough that reusing them is a false economy.

The automatic adjuster is the part that most often fails, usually by corroding solid. A seized adjuster means the shoes never take up wear and the pedal travel grows.

The wheel cylinder should be inspected at the same time: pull back the boot and look for fluid, because a weeping cylinder contaminates new linings immediately.

Handbrake cables and levers inside the drum should be checked for free movement while the drum is off, since access later means the same dismantling.

By the automatic adjuster if one is fitted and working, and manually to a specified clearance if not - it does not self-adjust the way a disc brake does.

A disc brake maintains its own clearance because the pads simply stay where the piston leaves them. A drum brake's shoes are pulled fully clear by springs every time.

That means clearance must be set, and it grows as the lining wears.

Most automatic adjusters work through the handbrake mechanism or during reverse braking, which is why an unused handbrake or a vehicle that never reverses can end up badly adjusted.

After fitting new shoes, adjust manually to the specified clearance rather than relying on the automatic mechanism to take up the whole difference.

Symptoms of poor adjustment are excessive pedal travel, an ineffective handbrake, or - if over-adjusted - dragging and a hot drum.

On heavy vehicles yes, where the shoe platforms are true and the economics are established. On light vehicles, exchange shoes are usually simpler.

Relining rivets or bonds new friction material to the existing steel shoe platform, which is often perfectly serviceable.

It is standard practice in commercial vehicle maintenance and is supported by established suppliers.

The platform must be checked for distortion and corrosion. A shoe that is no longer true will not contact the drum evenly however good the lining.

Rivet condition matters: a lining worn to the rivets scores the drum severely, which is why the minimum thickness above the rivet heads is the figure to watch.

For light vehicles the price difference between relined and new exchange shoes is usually too small to justify the handling, so exchange units dominate.

On many vehicles yes, through an inspection port in the backplate - and pedal travel and handbrake travel are indirect indicators.

A rubber-plugged inspection hole in the backplate lets the lining thickness be seen or measured without disturbing the drum.

It shows one point on one shoe, so it is a screening check rather than a full inspection.

Growing pedal travel and a handbrake that needs more clicks both indicate wear, though they also indicate a failed adjuster, so they are indirect.

Drums that have not been off for a long time seize onto the hub, so a full inspection has its own cost - which is exactly why the port exists.

Whatever the port shows, the full inspection is still needed at the scheduled interval, because springs, adjusters and cylinders cannot be seen through it.

A leading-trailing drum has one shoe helped by the drum's rotation and one opposed; a twin-leading drum has both helped, but only in one direction.

In a leading-trailing arrangement the leading shoe gets the self-servo effect and the trailing shoe does not, so the leading shoe wears faster.

That asymmetry is normal and expected, and it is why the two linings in a set are sometimes different.

A twin-leading layout uses two cylinders so both shoes lead in forward rotation, giving more braking force forwards and much less in reverse.

Duo-servo designs link the shoes so one drives the other, multiplying the effect further, and are common where a strong parking brake is needed.

The layout determines which shoe goes where, and fitting them the wrong way round is a real and common error - note the orientation before dismantling.