At an HGV MOT, officially the DVSA annual test, the brakes are assessed on a roller brake tester that measures each axle's brake effort, side-to-side balance and parking brake, followed by a visual check of the air system, hoses and pipes, brake chambers and the pads, discs or drums. How a defect is graded and what it means for the result depends on the current DVSA inspection manual.
In short:
- The roller brake tester measures each axle's brake effort and the difference between the left and right wheel.
- On air brakes, air leaks, warning lamps and the condition of the air lines are test items too.
- Common brake fails are imbalance, low brake effort, a weak parking brake and air leaks.
- Most of these defects show up in a simple check done a few days before the test.
What is checked on the brakes at an HGV MOT?
A brake check at the HGV annual test has two parts: measurement on the roller brake tester and a visual inspection under the vehicle. In Great Britain the test is carried out against the DVSA HGV inspection manual at a DVSA test station or an authorised testing facility. Defects are graded by severity; the current DVSA guidance sets out what each grade means for the result. Limit values are not given here.
The main brake items are:
- Service brake: Brake effort per axle, the difference between the left and right wheel, and how the effort varies as the wheel turns (ovality) are measured on the roller tester.
- Secondary and parking brake: Their performance is assessed as well; for the parking brake, the force applied by the spring brake chambers is measured.
- Air system: Pressure build-up, the low-pressure warning lamp and buzzer, and any audible leaks are checked.
- Visual inspection: Chafing and cracks on hoses, pipes and fittings; damage and corrosion on brake chambers and spring brakes; the condition of pads and discs or drums; the caliper dust boots.
- Electronics: The ABS or EBS warning lamp should go out after its self-check when the ignition is switched on.
What does the roller brake tester measure?
The vehicle is driven onto the rollers axle by axle, the wheels are turned and the brakes applied, and the tester records the brake effort at each wheel. The left-right difference, the variation in effort over each wheel revolution and the total brake effort are worked out from that.
On air brakes the braking force is set by the air pressure in the brake circuit, not by how hard the pedal is pressed. For that reason some stations fit a pressure gauge to the test points on the brake lines and judge the measured effort against the pressure at the time.
Load matters too. The braking power of a laden lorry is hard to measure directly when it is presented empty, because the load sensing valve (LSV, shown as ALB on many European vehicles) cuts the pressure sent to the rear axle on an empty vehicle. The result therefore depends on the valve setting and on how the station calculates it.
Whether the vehicle must be presented laden or unladen should be checked in the current DVSA guidance. On a multi-axle tractor unit or rigid, each axle is measured separately; a trailer has its own annual test.
- Brake effort per axle: Shows whether the total effort of the axle is adequate for the load it carries.
- Side-to-side difference: The difference in effort between the two wheels on one axle; it pulls the vehicle to one side under braking.
- Ovality: Effort that rises and falls as the wheel turns; it points to an out-of-round drum or a warped or uneven disc.
- Parking brake effort: Shows whether the spring brakes can hold the vehicle on their own.
The air system should be at normal working pressure; low pressure on the rollers distorts the readings.
Common HGV MOT brake fails
Common causes of brake fails on heavy vehicles are side-to-side imbalance, low brake effort, a weak parking brake, air leaks and warning lamps that stay on. The table matches each defect with what usually lies behind it.
| Defect found | Likely cause | Check before the test | What to do |
|---|---|---|---|
| Side-to-side brake imbalance | Pads worn or oiled on one side, seized caliper slide pins, a slack adjuster out of adjustment on drum brakes, a leaking brake chamber | Pads and discs or drums on both sides, free sliding of the caliper, the sound of leaks at chambers and hoses | Replace the pads or caliper parts; have the adjuster checked in a workshop |
| Low brake effort on an axle | Load sensing valve out of adjustment or a detached linkage, glazed linings, low system pressure, valve fault | LSV arm and linkage, the LSV setting plate on the vehicle, the pressure gauge | Have the LSV set to its plate or replaced in a workshop |
| Weak parking brake | Spring brake chamber, parking valve, or adjustment on drum brakes | Whether the vehicle holds on a safe slope with the parking brake applied; damage on the spring brake body | Have the spring brake and parking valve checked in a workshop |
| Air pressure dropping, audible leak | Leak at a fitting, hose, valve or brake chamber | With the wheels chocked and the engine off, leave the ignition on and the parking brake released; watch the gauge with the brake pedal pressed and released | Find the leak: how to find the leak in an air brake system |
| Brake effort fluctuating at the wheel (ovality) | Out-of-round drum, warped or cracked disc | Judder under braking, cracks on the disc face | Replace the disc or drum; wear limits come from the manufacturer's documentation |
| ABS/EBS or pad wear warning lamp on | Sensor, wiring or connector fault, pad wear sensor | Whether the lamp goes out after the ignition is switched on | Check the sensor, connector and wiring |
A continuous hiss from the exhaust port of the valve when the pedal is released can also be recorded as a defect. This usually points to an internal leak in the foot brake valve or the valves connected to it, and it should be diagnosed in a workshop.
How does the load sensing valve affect the HGV MOT brake result?
The ALB load sensing valve adjusts the brake pressure sent to the rear axle according to how heavily the vehicle is loaded; if its setting or linkage is wrong, the tester can show low or unbalanced brake effort.
On a mechanical load sensing valve, a lever and rod between the axle and the chassis sense the movement of the suspension. On air-suspension vehicles there are types that use the air pressure in the suspension bellows. On EBS vehicles load sensing is usually electronic and there may be no separate load sensing valve, so this varies with the vehicle and its equipment.
Visual checks before the test:
- The lever and rod are not broken, bent or detached.
- The joint moves freely without binding.
- No leak can be heard at the valve body and its connections.
- The setting plate (label) on the vehicle is in place and legible.
Setting values and procedures are not given here; the valve is set in a workshop against the plate on the vehicle and the manufacturer's service documentation. If the valve body leaks or the valve has an internal fault, choose a replacement from complete load sensing valves by matching the number of the old valve.
HGV walkaround check and pre-test brake checklist
A simple check a few days before the annual test shows most of the defects that cause a brake fail and leaves time for repairs. It overlaps with the driver's daily HGV walkaround check, in more detail. For what each component does in the system, see the main parts of an air brake system; the list below is only for test preparation.
| Check | How to do it | Watch for |
|---|---|---|
| Air tank draining | Open the drain valves and see whether water or oil comes out. | Water points to the air dryer and its cartridge; oil points to the compressor as well. |
| Pressure build-up and warning | With the engine running, watch the gauge rise and the warning lamp and buzzer go out. | Slow build-up is a sign of a compressor problem or a leak. |
| Listening for leaks | Chock the wheels; with the engine off, ignition on and parking brake released, listen for hissing first with the brakes released and then with the service brake applied, and watch the gauge. | The full method is in the leak article linked above. |
| Hoses, pipes, fittings | Look for chafing, cracks, crushing and loose clips. | Include trailer coupling heads and coiled air lines. |
| Brake chambers and spring brakes | Look for damage, corrosion and the sound of leaks on the bodies; on drum brakes check the push rod and slack adjuster. | The brake chamber (actuator) and the caliper dust boot are separate parts and must not be confused. |
| Pads and discs or drums | Check the wear indicator, cracks on the disc face and splits in the caliper dust boot. | Wear limits come from the manufacturer's documentation. |
| Warning lamps | When the ignition is switched on, the ABS/EBS and pad wear lamps should run their self-check and go out. | If one stays on, check the sensor and wiring. |
If the pad wear warning stays on, check the sensor connector and cable first. If the cable is chafed or broken, or the sensor has worn away with the pad, renew the caliper wear sensor with the correct part number for the vehicle.
Safety: Chock the wheels when working under the vehicle and never rely on the parking brake or air pressure alone. Do not open the body of a spring brake; the spring inside carries high force. Caging a spring brake with its release bolt, and replacing it, are workshop jobs done to the manufacturer's instructions. Release the system pressure before disconnecting any air line.
What to do after a brake fail: parts and servicing
The defect item in the test paperwork shows the axle and component concerned; the repair follows that item and the vehicle goes back for a retest. Retest rules and timescales come from the current DVSA guidance.
- 1. Note the defect by axle and side.
- 2. On an imbalance, assess the parts on both sides together; new pads fitted to one side only can widen the imbalance.
- 3. Have jobs such as load sensing valve setting and brake adjustment done in a workshop against the manufacturer's documentation.
- 4. After the repair, have the brakes verified on a controlled road test or on the roller tester.
When choosing parts, check the OEM or part number, the chassis number (VIN), the model year, the brake equipment (disc or drum, EBS or conventional) and the axle position together. In the MPT catalogue, a load sensing valve, foot brake valve and caliper wear sensor can be searched by OEM number.
Do not wait for the next test date to book a workshop if the pull or imbalance is also felt on the road, if the air pressure warning does not go out, or if the parking brake does not hold the vehicle.