Chapter 28

Truck Brake Systems

Objectives (1 of 3)

• Identify the components of a truck air brake system.

• Explain the operation of a dual-circuit air brake system.

• Understand what is meant by pneumatic and torque imbalance.

• Discuss the effects of the Federal Motor Vehicle Safety Standard No. 121 (FMVSS No. 121) on present-day air brake systems.

Objectives (2 of 3)

• Identify the major components of an air compressor.

• Describe the operation of desiccant and aftercooler-type air dryers.

• Outline the operating principles of the valves and controls used in an air brake system.

• Explain the operation of an air brake chamber.

• Outline the functions of the hold-off and service circuits in truck and trailer brake systems.

Objectives (3 of 3)

• Describe the operation of S-cam and wedge-actuated drum brakes.

• Describe the operating principles of manual and automatic slack adjusters.

• List the components and describe the operating principles of an air disc brake system.

• Describe the major components and operation of parking and emergency braking systems.

• Define the principles of brake balance.

Highway Tractor Air Brake System with ABS

Trailer Air Brake System with ABS

Air Brake Sub-systems

• The air supply circuit

• Primary circuit

• Secondary circuit

• Dash control valves–parking/emergency circuit

• Trailer circuit

• Foundation brakes

Pre-ABS Tractor, Trailer, and Dolly

Supply Circuit Components

Typical Air Compressors

Air Compressor Components

Air Compressor Operation

Governor

Shop Talk

• FMVSS No. 121 requires that governor cut-out be set within the range of 115 psi to 135 psi. The ATA TMC recommends that governor cut-out be set at 125 psi.

• It is a legal requirement that governor cut-in occur at no more than 25 psi less than governor cut-out pressure.

• If the difference between governor cut-out and cut-in is less than 20 psi, the result is too frequent cycling of the compressor loaded and unloaded cycles.

Typical Air Dryer Installation

Shop Talk

• Air dryers must be fitted with a safety valve.

• Because the system safety valve is located in the supply tank, if the hose from the dryer to the supply tank got plugged (with ice or contaminants) or kinked, the governor would not cut out the compressor effective cycle, and the resulting high pressure could explode the air dryer.

Warning

• Alcohol evaporators MUST be located downstream from the air dryer in the supply circuit.

• Alcohol will turn air dryer desiccant into mush if pumped through the system.

Air Dryer

Typical Air Reservoirs, Dump Valves,Check Valves, and Low Pressure Indicators

Auxiliary Air System Valves

An Older Truck’s Service and Parking Brake Circuits

Tractor Emergency Circuit Pre-ABS System

Pre-ABS Spring Brake Valve Circuit

Dash Control Valves

Shop Talk

• Make sure you understand the function of each dash control valve in the system.

• Block the wheels and get into a truck with a fully charged air system.

• Use the controls and observe the effect.

Tandem Axle Trailer Service and Parking/emergency Circuits

Gladhands

Caution

• When coupling a tractor to a trailer not equipped with spring brake chambers, always connect the gladhands and charge the trailer with air before attempting to couple the fifth wheel.

Foundation Brake Types

Cam Geometry

Brake Shoe Rollers

Q-brake

Wedge-actuated Brake System

Air Disc Brake

Dual Circuit Application/Treadle Valves

Typical Dual Circuit Brake Valve

Shop Talk

• One OEM survey of warranty returns indicates that over half of the foot valves submitted function perfectly.

• This misdiagnosis results from a failure by the technician to understand how the valve operates normally.

• Take some time to understand exactly how this important valve masters the operation of the service brakes.

Relay Valves

Primary Circuit

Quick Release Valve

Ratio Valves

Caution

• If a tractor is converted to a straight truck, the bobtail proportioning valve must be replaced with a non-proportioning relay valve.

• In a case in which a long-wheelbase tractor was converted to a wrecker, the operator complained of lazy service braking until the treadle was floored, when the brakes would lock. Replacement of the bobtail proportioning valve with a standard relay valve rectified the problem.

Caution

• Because air is delivered to the trailer hold-off circuit before the trailer tanks are charged, the trailer can be moved before any service braking is possible.

• You should charge trailer tanks before attempting to move the trailer.

Spring Brake Circuit

Shop Talk

• Current spring brake control valves must prioritize air to the hold-off circuit over air to the air tanks. – This means that when air builds up in the air system,

it is first delivered to the hold-off circuit, enabling the release of the parking brakes.

– This enables a vehicle with a disabled air supply system to be moved much more quickly. A much smaller volume of air has to be transferred to the vehicle before release of the parking brakes becomes possible.

Tractor Protection Circuit

Shop Talk

• FMVSS No. 121 requires that the TP valve isolate the tractor air supply from that of the trailer when pressure drops to between 20 and 45 psi.

In-line Single Check Valve

Double Check Valve

Air Brake Chambers

Cutaway of a Typical Air Chamber

Typical Spring Brake Chamber

Caution

• The spring force in a spring brake chamber can kill.

• Never take any risks with spring brake chambers.

Spring Brake Chamber Operation

Caution

• Spring brake chambers should be regarded as being potentially lethal.

• Be sure you read the setup and disarming instructions in Chapter 31, and always observe the OEM precautions.

Manual and Automatic Slack Adjusters

Stroke-sensing Automatic Slack Adjuster

Shop Talk

• Slack adjusters without a grease fitting should not be assumed to be functional.

• Slack adjusters should be lubricated with a low-pressure grease gun.

Note

• Around 95 percent of braking on a loaded tractor/trailer combination involves brake application pressures of 25 psi or less; 85 percent of braking involves brake application pressures of 15 psi or less.

Summary (1 of 4)

• An air dual-circuit brake system is composed of a supply circuit, primary circuit, secondary circuit, parking/emergency control circuit, trailer circuit, and foundation brake assemblies.

• Air compressors are single-stage, reciprocating piston air pumps that are either gear- or belt-driven.

• Air dryers are used to help eliminate moisture and contaminants from the truck’s air system.

Summary (2 of 4)

• Dual-circuit application or foot valves, trailer application valves, bobtail proportioning valves, ratio valves, quick-release valves, relay valves, TP valves, dash control valves, double check valves, and check valves are some of the critical valves of an air brake system.

• The potential energy of compressed air is changed into mechanical force in an air brake system by slack adjusters and brake chambers.

• The most common type of foundation brake assembly used on air brake-equipped trucks is the S-cam type.

Summary (3 of 4)

• Slack adjusters multiply the force applied to them by the brake chamber into brake torque.

• Brake torque applied to the S-camshafts results in the shoes being forced against the drum.

• Air discs operate by using an air-actuated caliper to squeeze brake pads against both sides of a rotor.

• Wedge brakes use a drum, a pair of shoes, and air-actuated wedges are used to force the shoes against the drum.

Summary (4 of 4)

• Brake torque balance refers to the ability of a brake system to apply balanced mechanical brake force at all the foundation brake assemblies.

• Pneumatic timing refers to the ability of an air brake system to time the air control and actuation circuits so all the foundation brake components are applied at exactly the same moment.