What is the mechanism of a truck’s brakes?

Braking systems on current heavy vehicles are supplemented by safety systems such as ABS (anti-lock braking system), EBS (electronic braking system), ESC (electronic stability control), and ASR (automated stability control) (anti-slip regulation).

A pneumatic (air) system is typical for medium and heavy trucks. This system includes the following components:

  • brakes in service
  • brakes for parking
  • valve with a treadle (brake pedal)
  • Relay valves for safety
  • compressor for air
  • tanks of air

The parking brake is a disc or drum held in place by spring pressure through a brake pot. The spring brake’ is released when the engine is started because air pressure increases. The brakes are activated by default if there is no air pressure, preventing a disconnected trailer from escaping.

When the brake pedal is pressed, the servicing brakes are activated. This sends pressurized air to the brake pot, causing it to engage. The pressure in this air is usually between 100 and 120 psi.

Brakes are roller-tested to ensure that they function properly. This entails driving one axle at a time over a set of rollers to assess braking efficiency and imbalance. The relationship between the weight on the axle being tested and overall vehicle brake force is measured by braking efficiency.

An ‘imbalance’ occurs when the braking performance on the right and left sides of the axle differs. A maximum imbalance of 30% is permitted; otherwise, the vehicle will fail the test.

Energy conversion from one form to another

Energy can’t be generated or destroyed; it can only be transformed from one state to another. Brakes, for example, use friction to reduce the rotation of the wheel, converting motion, or kinetic energy, into heat (thermal energy). The more kinetic energy there is, the faster the vehicle is traveling, and the kinetic energy is quadrupled when the speed is doubled.

Brake fading occurs when the brakes overheat. This can happen if the brake fluid is boiled or the overheated brake components. When brake components, such as discs and pads, become too hot, a superheated film of gas forms, reducing the amount of friction the brake can offer. The entire disc can shatter in extreme circumstances, causing a pulsing sensation through the brake pedal. If the brake fluid boils, the pressure required to apply the brakes is lost, and the pedal may drop to the floor.

The retarder, not the service brakes, should be used to slow the truck down. The truck’s service brakes are employed to bring it to a complete stop.


A dragging brake is often indicated by a strong odor and excessive heat from the wheel. This can cause the wheel hub to heat up, which can be detected after a long drive if one corner is warmer than the other on the opposite side.

If you see oil near the braking components, you should immediately take action.

When you hear a metallic sound when you brake, it usually means the pads or drums have worn too much, and now it’s metal on metal. There is an immediate need for assistance.

Brake imbalance is indicated by the car pulling to one side while braking. This must be investigated as quickly as feasible.