5.2 Braking Systems, Air Pressure & Tyres

Full Air-Brake System Fundamentals

A full air-brake system stores compressed air and meters it to brake actuators when the driver uses the service brake. Typical components include the compressor, governor or electronic control, air dryer, reservoirs, protection valves, foot valve, relay valves, chambers, foundation brakes and dashboard gauges or warning devices. ABS limits wheel lock; EBS can transmit commands electronically while retaining pneumatic energy.

Pressure figures are vehicle-specific. The manufacturer states the normal build-up range, governor cut-in and cut-out points, minimum operating pressure and the point at which audible and visual warnings must activate. Do not memorise 8–10 bar working pressure or 5–6 bar warning pressure as universal Category D law. The correct theory principle is to know the vehicle's handbook and react immediately to its warnings.

Pre-use Brake Checks

Before moving into traffic, observe that pressure builds in the expected time and range, warnings cancel at the specified pressure, gauges agree where dual circuits are shown, and no abnormal leakage is heard. With the area clear, carry out the operator's prescribed service-brake and parking-brake checks. Warning lamps for ABS, EBS or stability systems must complete their normal self-test. A warning that remains on, slow pressure build, rapid loss, pulling, poor response or an audible leak is a defect.

If a low-pressure warning activates on the road, slow and stop in a safe place using controlled braking while adequate pressure remains. Apply the parking brake when stationary, contact the operator and do not continue until the cause is resolved. Never repeatedly pump the pedal: each application consumes stored air.

Reservoirs, Moisture and Spring Brakes

Compression creates heat and moisture. Air dryers and automatic drains reduce contamination; some vehicles also require manual reservoir draining at intervals. Follow the actual system and maintenance schedule. “Drain every tank daily” is not a safe universal instruction because the design and procedure differ, and opening a drain incorrectly can be hazardous.

Powerful springs apply the parking brake when air is released from the spring-brake chambers. Air pressure normally compresses those springs to release the brake. This fail-safe principle helps secure the vehicle if pressure is lost, but an uncontrolled spring-brake application can destabilise a moving bus. Treat pressure loss as an emergency and never tamper with or mechanically cage a spring brake unless qualified and following the repair procedure.

Retarders provide supplementary slowing and reduce service-brake heat on long descents, but they do not replace the service brake or a suitable low gear. Reduce or avoid strong retarder use on slippery roads if it could break drive-axle adhesion.

Tyre Safety, Tread Depth & Legal Regulations for Category D

Tyres represent the sole contact patch between a 15-tonne bus and the road surface:

• Legal Minimum Tread Depth: Under Irish road traffic regulations, the minimum legal tyre tread depth for Category D passenger vehicles is 1.6 mm across the central three-quarters of the tread width around the entire circumference of the tyre.
• RSA Recommended Tread Standard: Although 1.6 mm is the legal absolute minimum, the Road Safety Authority strongly recommends a minimum tread depth of 3.0 mm for heavy commercial passenger vehicles. Higher tread depth is essential for dispersing water efficiently, preventing hydroplaning, and ensuring adequate wet-weather braking performance.
• Inflation Pressure & Sidewall Inspections: Tyres must be inflated to the manufacturer's specified cold pressure (often exceeding 8 to 9 bar for heavy bus tyres). Over-inflated tyres cause central tread wear and reduced traction; under-inflated tyres cause rapid shoulder wear, excessive heat buildup, and severe blowouts. Sidewalls must be inspected for cuts, bulges, cracks, or exposed textile/steel cords. Any tyre displaying sidewall cord exposure or structural bulging must be removed from service immediately.
• Regrooved Tyres: Regrooving (cutting deeper tread grooves into worn tyres) is strictly regulated. Regrooving is legally permitted on Category D vehicles ONLY if the tyre was specifically designed and molded by the manufacturer for regrooving, clearly stamped with the word "REGROOVABLE" on the sidewall, and performed by a certified technician. Regrooving non-regroovable tyres is illegal.
• Dual Wheel Fitting: On rear dual-wheel axles, both inner and outer tyres must be of matching size, construction type (radial), and inflation pressure, with no stones or debris wedged between the dual wheels.

Summary Table: Pneumatic System Pressures & Tyre Specifications