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Frequently Asked Questions
Which brake parts should be replaced together?
Friction parts always across an axle, and discs or drums in pairs whenever they are replaced at all.
Braking force must be equal on both sides of an axle. New friction on one side and half-worn friction on the other pulls the vehicle under braking, which is a handling problem before it is a wear problem.
Discs and drums follow the same logic. A new disc on one side and a thin, glazed one on the other gives different friction and different heat capacity at the same pedal pressure.
Hardware - clips, shims, springs and pins - is normally supplied with the friction kit and should be fitted rather than reused. Most pad noise complaints trace back to reused hardware.
Caliper slide pins and their boots need cleaning and regreasing at every pad change, which is the single highest-value ten minutes in the job.
Front and rear axles are independent. There is no requirement to do all four corners at once unless they have genuinely worn together.
How do I tell a friction problem from a hydraulic one?
By what the pedal does. A hard pedal with poor stopping is friction; a pedal that travels too far or sinks under steady pressure is hydraulic.
Worn, glazed or contaminated friction material gives a firm pedal and a vehicle that does not slow as it should. The pedal feels normal because the hydraulics are fine.
A pedal that goes closer to the floor than it used to, or that sinks slowly while held, indicates fluid going somewhere it should not - an internal master cylinder leak, an external leak, or air in the system.
A spongy pedal that firms up on a second press is air, and air means the system has been opened or a seal is drawing in.
Pulling to one side under braking is usually a seized caliper or a collapsed flexible hose acting as a one-way valve, not a friction problem.
Vibration through the pedal at speed is disc thickness variation, which is geometry rather than either category.
When do discs and drums have to be replaced rather than machined?
When machining would take them below the minimum thickness or beyond the maximum diameter cast into the part - and increasingly the answer is simply to replace them.
Every disc carries a minimum thickness and every drum a maximum internal diameter. Those figures are the point at which the part can no longer absorb a hard stop's heat safely.
Machining removes material, so a disc must be thick enough that it will still be above the minimum after the cut and after the next set of pads has worn.
On many modern light vehicles the discs are thin enough new that there is no useful machining allowance at all, and replacement is the only option.
Heavy vehicle drums and discs generally do have an allowance, and machining is normal practice where the equipment exists.
Measure rather than judge by eye. A lip at the outer edge shows wear has happened but says nothing about how much thickness remains.
How often should brake fluid be changed?
On the manufacturer's schedule regardless of mileage, because the fluid degrades by absorbing water rather than by being used.
Conventional brake fluid is hygroscopic: it draws moisture from the air through hoses, seals and the reservoir cap over time.
Water lowers the boiling point. Fluid that boils under hard or repeated braking puts vapour in the lines, and vapour compresses - which is a pedal that goes to the floor at exactly the wrong moment.
Water also corrodes cylinder bores and internal components from the inside, which is a slow, expensive failure.
Test strips and electronic testers give a moisture reading in minutes and are worth having in any workshop.
Fluid types are not freely interchangeable - the glycol-based grades and silicone-based fluid behave differently and must not be mixed. Use what the vehicle specifies.
Are aftermarket brake parts as good as original equipment?
The good ones are, and the difference between good and bad is documented approval rather than price or appearance.
Most markets operate a type-approval or homologation regime for replacement friction material, requiring it to perform within a defined band of the original part.
Parts carrying that approval are a genuine alternative. Parts without it may be fine and may equally be untested, and there is no way to tell by looking.
The approval regimes differ by territory, so a part legal in one market is not automatically legal in another - which matters for fleets operating across borders.
Beyond legality, compound choice affects noise, dust, cold performance and disc wear, and a cheaper pad frequently costs more over its life by wearing the disc.
For fleets, standardising on one approved supplier per platform makes performance predictable and simplifies stockholding far more than chasing unit price.
What causes brake noise, and does it matter?
Most squeal is vibration rather than a fault, but grinding, knocking and a rhythmic noise are all genuine problems.
High-frequency squeal is the pad vibrating against the disc. Anti-squeal shims, correct hardware and a thin film of the right lubricant on the pad backing usually cure it.
Grinding means the friction material is gone and metal is contacting the disc. That is a stop-now fault, and it usually means a new disc as well as new pads.
A knock on first application or on direction change is normally loose hardware, worn slide pins or excessive pad-to-caliper clearance.
A rhythmic noise that changes with road speed rather than pedal pressure points at the disc - run-out, thickness variation, or a wheel bearing.
New pads are often slightly noisy until bedded in. Persistent noise after the bedding-in procedure is a fault, not a characteristic.
What should a fleet hold in stock?
Friction and hardware for the platforms it runs, plus flexible hoses and a caliper repair kit per common caliper - not complete assemblies.
Pads and shoes are predictable consumables and are the obvious stock item, held by platform rather than by vehicle.
Discs are bulkier and turn over more slowly, but holding a pair for the highest-volume platform turns a two-visit job into one.
Flexible hoses fail on age and corrosion rather than mileage, and a vehicle off the road waiting for a hose is a poor use of a workshop bay.
Caliper repair kits are cheap and let a seized caliper be recovered rather than replaced, where the caliper body itself is sound.
Brake fluid in sealed containers, and a rule that opened containers are used or discarded rather than stored - an opened bottle absorbs moisture in exactly the way the system does.