Brake Systems and Air Supply

Introduction to Air Brake Systems

The air brake system is the most critical safety device on a heavy vehicle combination (tractor-trailer). Unlike hydraulic systems, compressed air is used as the force transmission fluid. This choice is based on three main reasons: air is inexhaustible, it does not freeze at low temperatures, and a leak in the system triggers an emergency braking (safety braking) rather than a total loss of braking.

The Canadian Electrical Code, Chapter V (CE Code V) governs the installation of air brake systems on road vehicles in Canada. This code, developed by the Canadian Standards Council (CSC), is the mandatory normative reference for the inspection and certification of heavy vehicles. The Canada Motor Vehicle Safety Regulations (CMVSR) also apply, but CE Code V is specific to brake systems.

Fundamental Physical Principles

Atmospheric pressure at sea level is approximately 101.3 kPa (14.7 psi). Pneumatic systems operate by creating a pressure difference between compressed air (typically between 550 and 1,035 kPa) and atmospheric pressure. The force exerted by compressed air is calculated using the formula:

Force (N) = Pressure (Pa) × Area (m²)

For example, a brake diaphragm with an effective area of 0.05 m² subjected to a pressure of 620 kPa (90 psi) generates a force of:

0.05 × 620,000 = 31,000 N (approximately 3,160 kgf)

This force is then multiplied by the slack adjuster lever and the cam mechanism (or the piston pushrod in air disc brakes) to produce the braking torque at the wheels.

Units of Measurement and Conversions

Unit	Symbol	Equivalence
Pascal	Pa	1 N/m²
Kilopascal	kPa	1,000 Pa
Bar	bar	100 kPa
Pound per square inch	psi	6.895 kPa
Megapascal	MPa	1,000 kPa

Quick conversion: 100 psi ≈ 690 kPa ≈ 6.9 bar. For the exam, remember that normal service pressure is 620 to 690 kPa (90 to 100 psi).

Air Supply Circuit Components

Compressor

The compressor is driven by the vehicle's engine via a belt or gear. It draws in ambient air, compresses it, and discharges it into the system. The compressors used are piston type (single or double acting) or screw type. Output is expressed in liters per minute (L/min) or cubic feet per minute (CFM).

The compressor is equipped with a pressure regulator (governor) that controls the operating range. The typical cycle is:

Cut-out: 1,035 kPa (150 psi)

Cut-in: 830 kPa (120 psi)

This 205 kPa (30 psi) range is called the governor differential. The governor can be mounted on the compressor or remotely, and it controls the unloader valve that puts the compressor on load or unloaded.

Unloader Valve

The unloader valve has two functions:

24.Unloading: when pressure reaches the cut-out threshold, the valve opens the compressor intake to the atmosphere, preventing compression.

25.Initial unloading: at startup, it allows the compressor to run without load until the governor switches to load mode.

Air Dryer

The air dryer is a mandatory component on vehicles manufactured after 1995 in Canada. It removes moisture and contaminants from the compressed air. Two technologies exist:

Desiccant type: air passes through a bed of silica gel or activated alumina granules that adsorb moisture. The dryer is regenerated by a periodic purge (regeneration cycle) that expels accumulated water.

Membrane type: uses a semi-permeable membrane that separates water vapor from the compressed air.

The dryer is equipped with an electric heater (12 V or 24 V element) to prevent the purge valve from freezing in winter. The purge valve opens automatically when the governor switches to unload mode, expelling water and contaminants.

Air Reservoirs

Reservoirs (or tanks) store compressed air. They are made of welded steel and must comply with CSA B51 (boilers, pressure vessels, and pressure piping). Each reservoir is fitted with:

A drain cock (drain valve) at the lowest point

A safety valve set at 1,380 kPa (200 psi) or 150% of the maximum service pressure

A pressure indicator (gauge) on the primary reservoir

The minimum total reservoir capacity is regulated: it must allow the system to operate for a specified number of brake cycles without compressor input (see Calculations section).

Circuit Protection Valve (Safety Valve)

The protection valve (or reservoir protection valve) is a check valve that isolates the circuit in the event of a major leak. It is set to open at a pressure slightly below normal service pressure (approximately 620 kPa). In the event of a circuit rupture, the valve closes, preserving pressure in the intact circuits.

Brake Circuits

General Architecture

The air brake system is divided into independent circuits to ensure residual braking in the event of failure. The standard architecture includes:

42.Service circuit (normal braking): controlled by the brake pedal

43.Parking/secondary circuit: controlled by the parking valve

44.Emergency circuit: activates automatically in the event of pressure loss

The foot valve (brake pedal) is a proportional regulator that modulates the pressure sent to the brake chambers based on the force applied by the driver. It is dual-circuit: two independent sections supply the front and rear axles respectively.

Relay Valves

Relay valves are used on remote axles (rear axles and trailers) to reduce response time. They receive a pilot signal from the foot valve and use air from the local reservoir to supply the brake chambers. This avoids the propagation delay of air over long distances.

The relay ratio is typically 1:1, but some valves have a differential ratio (e.g., 2:1) for special applications.

Proportioning Valves

The proportioning valve (or distribution valve) modulates braking pressure based on axle load. On vehicles with air suspension, the valve is linked to ride height by a linkage. It reduces pressure to the rear axles when the vehicle is empty, preventing wheel lock-up.

Brake Chambers

Brake chambers convert pneumatic pressure into mechanical force. Two main types:

Diaphragm chamber: a rubber diaphragm deforms under pressure and pushes the pushrod. Maximum pushrod stroke: 38 to 64 mm depending on type.

Piston chamber: a piston slides within a cylinder. It offers a longer stroke and better durability.

Chambers are classified by size (effective diaphragm area): types 9, 12, 16, 20, 24, 30. The number corresponds to the area in square inches (e.g., type 30 = 30 in² ≈ 193 cm²).

Spring Brake Chambers (Combination Chambers)

Spring brake chambers incorporate a powerful spring for parking and emergency braking. The spring is compressed by compressed air during normal operation. In the event of pressure loss, the spring extends and applies the brakes.

Critical data:

Spring force: can reach 4,500 N at full stroke

Release pressure: approximately 450 kPa (65 psi) to begin compressing the spring

Service pressure: 620 kPa (90 psi) to keep the spring fully compressed

The cage bolt allows mechanical compression of the spring for towing or maintenance. This operation is extremely dangerous: the spring can eject the bolt with deadly force. The safe procedure requires securing the chamber in a vise or holding device before unscrewing.

Air Disc Brakes

Increasingly common on new trailers, air disc brakes use a screw and nut mechanism to convert pneumatic force into clamping force. They offer:

Better heat dissipation

Faster response

Integrated self-adjustment

The operating clearance is maintained automatically by an adjustment mechanism, eliminating the need for manual cam adjustment.

Trailer Brake System

Trailer Control Valves

The trailer is supplied by two lines:

Supply line (service line): blue, supplies the trailer reservoirs

Control line (brake line): red, transmits braking pressure

The trailer control valve is mounted on the tractor. It combines the functions of:

Supplying the trailer reservoirs

Controlling the trailer brakes

Emergency braking in the event of supply line rupture

Trailer Service Valve (Emergency Relay)

The emergency relay valve (or trailer protection valve) is mounted on the trailer. It has two main functions:

80.Relay function: transmits control line pressure to the trailer brake chambers

81.Emergency function: in the event of a pressure drop in the supply line (rupture or disconnection), the valve automatically applies the trailer brakes using air stored in the trailer reservoirs

The emergency trigger pressure is approximately 280 to 380 kPa (40 to 55 psi). Below this threshold, the valve closes and applies the brakes.

Trailer Protection Valve (Tractor)

The protection valve mounted on the tractor protects the tractor circuit in the event of a rupture in the trailer supply line. It closes automatically when pressure drops below approximately 380 kPa (55 psi), preserving pressure for the tractor brakes.

Gladhands (Couplings)

Gladhands are the coupling fittings between the tractor and trailer. They are color-coded:

Red: service line (brake)

Blue: supply line (air)

Gladhands must be clean, lubricated, and free of damage. A defective O-ring is a common cause of leaks. When coupling, the two halves must be pushed together and rotated 90° to lock.

Trailer Parking Brake

The trailer parking brake is applied by the springs in the spring brake chambers. The parking valve (dash valve) on the tractor dashboard controls the exhaust of air from the spring chambers. When the valve is pulled out, air is exhausted and the springs apply the brakes.

On some trailers, a pneumatically controlled parking valve (push-pull) is mounted on the trailer itself, allowing the trailer brakes to be released without the tractor (e.g., for loading).

Brake Adjustment and Maintenance

Cam Brake Adjustment

Adjustment of S-cam brakes is critical to performance. Pushrod stroke must be checked with the brakes applied at 620 kPa (90 psi). The maximum limits are:

Chamber Type	Maximum Allowable Stroke
Type 9	38 mm (1.5 in)
Type 12	38 mm (1.5 in)
Type 16	44 mm (1.75 in)
Type 20	44 mm (1.75 in)
Type 24	51 mm (2 in)
Type 30	51 mm (2 in)

Adjustment is made by turning the slack adjuster on the cam lever. The correct procedure:

98.Ensure the brakes are released

99.Turn the adjuster in the tightening direction until the shoes contact the drum

100.Back off a quarter turn (or according to manufacturer specifications)

101.Check the stroke with the brakes applied

Important: excessive adjustment (stroke too short) can cause brake drag and overheating. Insufficient adjustment (stroke too long) reduces braking force and can damage the chamber.

Lining and Drum Inspection

The minimum thickness of brake linings is 3.2 mm (1/8 in) at the thinnest point. The drum must be measured at several locations; maximum wear is typically 4.8 mm (3/16 in) relative to the nominal diameter. Surface roughness must not exceed 0.025 mm (0.001 in) after machining.

System Drainage

Draining water from the reservoirs must be done daily. Reservoirs are equipped with manual drain valves (pull cable) or automatic (self-draining) types. A defective automatic drain valve can allow water to accumulate and freeze in winter, blocking the valves.

Calculations and Specifications

Reservoir Capacity

The total reservoir capacity must be sufficient to allow a minimum number of full brake applications without compressor input. CE Code V requires that the system be capable of at least 8 full brake applications with a pressure drop not exceeding 140 kPa (20 psi) per application.

Calculation example:

Initial pressure: 830 kPa (120 psi)

Minimum final pressure: 550 kPa (80 psi)

Maximum total drop: 280 kPa (40 psi)

Drop per brake application: 35 kPa (5 psi) maximum

The volume of air consumed per brake application depends on the number of chambers, their size, and pushrod stroke.

Pressure Build-Up Time

The compressor must be able to raise pressure from 0 to 690 kPa (0 to 100 psi) within a maximum time of:

Road vehicles: 5 minutes at idle

Vehicles with trailer: 7 minutes at idle

The build-up time from 550 to 690 kPa (80 to 100 psi) must not exceed 2 minutes at idle.

Allowable Leaks

CE Code V specifies maximum leak rates:

Tractor alone: pressure loss not exceeding 14 kPa (2 psi) in 1 minute, engine off, brakes released

Tractor + trailer: loss not exceeding 21 kPa (3 psi) in 1 minute

With brakes applied: loss not exceeding 21 kPa (3 psi) in 1 minute

Response Times

Response times measured from pedal activation:

Tractor: rise to 75% of maximum pressure in less than 0.5 seconds

Trailer: rise to 75% in less than 0.6 seconds

Release: drop to 10% of pressure in less than 0.6 seconds

Canadian Standards and Regulations

Canadian Electrical Code, Chapter V

CE Code V is the national standard for road vehicle brake systems. Relevant sections include:

Section 1: Definitions and general requirements

Section 2: Service brake requirements

Section 3: Parking brakes

Section 4: Air supply systems

Section 5: Performance requirements

Key CE Code V requirements:

Every vehicle must be equipped with a service brake system acting on all wheels

The parking brake must hold the vehicle on a 20% grade (11.3°)

Systems must be designed for a service pressure of 690 kPa (100 psi)

Reservoirs must comply with CSA B51

Canada Motor Vehicle Safety Regulations (CMVSR)

The CMVSR, under the Motor Vehicle Safety Act, establishes manufacturing standards. CMVSS 121 (Canada Motor Vehicle Safety Standard) covers air brake systems. Requirements include:

Braking performance on dry and wet roads

Resistance to fade

Circuit integrity in the event of failure

Complementary CSA Standards

CSA B51: Design and manufacture of pressure vessels

CSA B620: Fuel tanks and fuel systems

CSA D426: Requirements for heavy vehicle brake systems (repair standard)

Inspection and Diagnostic Procedures

Daily Pre-Trip Inspection

The daily pre-trip inspection is mandatory for all heavy vehicle drivers. Brake system check points:

156.Visible leaks: check fittings, chambers, valves

157.Compressor oil level (if separately lubricated)

158.Reservoir drainage: open drain valves and check for water

159.Service pressure: check the gauge (minimum 690 kPa)

160.Low pressure warning: the warning light must illuminate below 410 kPa (60 psi)

161.Parking brake: apply and verify holding

162.Pushrod stroke: measure with brakes applied

Static Leak Test

Procedure:

165.Engine off, brakes released

166.Note the initial pressure (must be ≥ 690 kPa)

167.Wait 1 minute

168.Note the final pressure

169.The drop must not exceed 14 kPa (2 psi) for a tractor alone

Leak Test with Brakes Applied

171.Engine off, pressure at 690 kPa

172.Apply the brakes fully

173.Wait 1 minute

174.The drop must not exceed 21 kPa (3 psi)

Common Fault Diagnosis

Symptom	Probable Cause	Verification
Pressure does not build	Defective compressor, slipping belt, stuck governor	Check belt, compressor discharge
Pressure drops rapidly	Leak in circuit, safety valve open	Leak test, inspect fittings
Brakes dragging	Excessive adjustment, stuck relay valve, broken return spring	Check pushrod stroke, drum temperature
Weak brakes	Excessive stroke, worn linings, insufficient pressure	Measure stroke, check pressure
Trailer brakes not applying	Defective control valve, obstructed control line, incorrectly connected gladhands	Check pressure in red line

Pitfalls to Avoid

178.Confusing the red and blue lines: the red line is the control line (brake), the blue is the supply line. Reversing them causes inoperative braking or permanent emergency braking.

179.Forgetting purge time: the air dryer must be purged regularly. A purge valve blocked by ice is a common cause of winter breakdowns.

180.Neglecting the spring chamber cage bolt: always use a restraining device. Spring force can project the bolt with deadly energy.

181.Confusing service pressure and cut-out pressure: normal service pressure is 690 kPa (100 psi), but the compressor cuts out at 1,035 kPa (150 psi). Dashboard gauges indicate reservoir pressure, not brake pressure.

182.Ignoring pushrod stroke limits: excessive stroke (beyond the table values) indicates a need for adjustment or worn components. Never exceed manufacturer limits.

183.Using non-certified parts: brake components must comply with CE Code V. Counterfeit parts can cause catastrophic failures.

184.Forgetting the leak test after repairs: any work on the pneumatic circuit must be followed by static and dynamic leak tests.

185.Confusing chamber types: type 30 chambers have a maximum stroke of 51 mm, not 38 mm. Check the label on the chamber.

186.Neglecting drum condition: a cracked or warped drum can shatter in service. Always measure the diameter and check for cracks.

187.Not checking the protection valve operation: in the event of a trailer supply line rupture, the valve must close to preserve tractor pressure. Periodic testing is mandatory.

Summary

The air brake system is a complex assembly of interdependent components that must work in perfect harmony. The essential points to remember for the exam:

Service pressure: 690 kPa (100 psi); compressor cut-out at 1,035 kPa (150 psi)

Three circuits: service, parking, emergency — each must be independent

Spring brake chambers: the spring applies the brakes in the event of pressure loss; the cage bolt is dangerous

Emergency relay valve: triggers at 280-380 kPa (40-55 psi)

Maximum strokes: 38 mm (types 9-16) to 51 mm (types 24-30)

Allowable leaks: 14 kPa/min tractor alone, 21 kPa/min with trailer

Build-up time: 5 minutes (tractor) or 7 minutes (with trailer) to reach 690 kPa

Standards: CE Code V, CMVSR (CMVSS 121), CSA B51

Daily reservoir drainage to prevent freezing and contamination

Mastery of these concepts, calculations, and inspection procedures is essential to pass the Red Seal exam. Questions often focus on limit values, safety procedures, and fault diagnosis. Practice identifying components on schematics and calculating forces and pressures.

Safety is paramount: a defective brake system can cause serious accidents. The technician must always verify their work through functional tests and document interventions in accordance with regulatory requirements.

Brake Systems and Air Supply