Hold firm pressure on the pedal for around a minute with the engine running. A pedal that slowly sinks with no external leak is an internal master cylinder leak.

The test works because an internal leak lets fluid pass the piston seal continuously while pressure is applied.

Check for external leaks first - at the cylinder body, the servo face behind it, the unions and every flexible hose.

Fluid disappearing from the reservoir with no visible leak anywhere often means it is leaking rearwards into the servo, which is worth checking by removing the cylinder.

A pedal that is firm but travels further than it used to may be air, or may be excessive drum brake clearance rather than the cylinder.

If in doubt, isolate circuits with clamps or blanking plugs to establish which part of the system is losing pressure.

It keeps part of the braking working. The pedal travels much further and stopping distances lengthen substantially, but the vehicle still stops.

The master cylinder contains two pistons in series, feeding two independent circuits, usually split either front and rear or diagonally.

If one circuit loses pressure, the corresponding piston travels to its stop and the other circuit still functions.

A diagonal split keeps one front and one rear brake on each circuit, which preserves more balance than a front-rear split when half is lost.

The long pedal travel and the warning lamp are the driver's signal that this has happened, and it is not a condition to keep driving with.

Dual circuits are a legal requirement in essentially every market, and defeating or bridging them during repair is both dangerous and non-compliant.

Because the leaking fluid runs straight onto the friction linings, and contaminated linings cannot be cleaned.

Brake fluid soaks into the porous friction material and destroys its friction characteristics permanently.

Washing or wire-brushing removes the surface but not what has penetrated, and the shoe will grab or fade unpredictably.

Contamination also causes the classic pull to one side, because only one drum is affected.

That is why pulling back the wheel cylinder boots is part of every drum service: a trace of fluid caught early means one cylinder, not a cylinder and a set of shoes.

The same applies to disc brakes with a leaking caliper - contaminated pads are replaced, not cleaned.

Only if the bore is clean. Corrosion pitting is what usually kills them, and a kit will not seal a pitted bore.

Repair kits containing seals, boots and sometimes pistons are available and are worthwhile on heavy or specialist vehicles.

The decision is made by inspecting the bore. Light staining can be cleaned; pitting cannot be sealed against.

Honing to remove pitting enlarges the bore and removes the finish, and is not a repair on modern cylinders.

Since the underlying cause is nearly always moisture in the fluid, a rebuilt cylinder in a system with old fluid will fail again.

On light vehicles the price of a new wheel cylinder rarely justifies a rebuild.

In the manufacturer's sequence, keeping the reservoir topped, and with the correct procedure for the vehicle's ABS unit.

Bleeding order matters because it determines whether air is pushed out or moved around the system; it is not always furthest-from-master-first on modern vehicles.

The reservoir must never be allowed to empty, or air enters the master cylinder and the job starts again.

A new master cylinder should be bench-bled before fitting, which removes most of the air before it can be pushed into the lines.

ABS modulators trap air that a conventional bleed cannot reach. Many vehicles require a diagnostic tool to cycle the pump and valves during bleeding.

Finish with a firm-pedal check and a road test at low speed before the vehicle returns to service, and always use fresh fluid from a sealed container.

Usually not - a hard pedal with poor braking points at the servo or its vacuum supply, not at the master cylinder.

The master cylinder generates pressure in proportion to the force applied to it. If that force is not being multiplied, the pedal feels hard and the vehicle stops poorly.

On a petrol engine the servo draws vacuum from the inlet manifold; a perished hose or a failed non-return valve removes the assistance.

Diesel and many modern engines use a dedicated vacuum pump, which is itself a wear item.

The standard check is to pump the pedal with the engine off to exhaust the reserve, hold it down and start the engine. The pedal should sink noticeably as vacuum arrives.

A ruptured servo diaphragm can also draw brake fluid rearwards from the master cylinder, which explains fluid disappearing with no visible leak.

It reduces the pressure sent to the rear brakes as the vehicle's load falls, so an unladen vehicle does not lock its rear wheels.

Braking transfers weight forwards, and an unladen commercial vehicle has very little weight on the rear axle to begin with.

Without limiting, the rear brakes would lock early and the vehicle could swing.

A mechanical valve is linked to the axle or suspension and senses ride height as a proxy for load, which is why suspension changes affect braking balance.

Electronic brake force distribution performs the same job through the ABS system on modern vehicles, with no mechanical linkage to seize or go out of adjustment.

Where a mechanical valve is fitted, its linkage needs checking and its setting verifying after any suspension work - it is frequently overlooked and it silently changes how the vehicle stops.