Air Brakes Fundamentals

The primary advantage of air brake systems on heavy vehicles is that air:

Can be compressed and stored, allowing multiple brake applications without direct mechanical linkage Is lighter than hydraulic fluid and requires no compressors Cannot leak from lines or fittings Does not require any maintenance or drying

In a basic air brake system, the compressor’s main function is to:

Apply the foundation brakes directly Compress atmospheric air and supply it to the reservoirs Control ABS operation Release the parking brakes

The governor in an air brake system is used to:

Control brake chamber stroke length Regulate compressor cut-in and cut-out pressures Lubricate the compressor Adjust foundation brake clearance

The term 'cut-out pressure' refers to the:

Pressure at which the hand valve operates the trailer brakes Pressure at which the governor unloads the compressor Minimum pressure required to release spring brakes ABS activation threshold

The primary (service) reservoirs in an air brake system are used to:

Store hydraulic fluid for backup braking Store compressed air for service brake applications Vent excess air to atmosphere Lubricate brake components

Most air brake components are rated using pressure units of:

Volts (V) Kilowatts (kW) Kilopascals (kPa) or pounds per square inch (psi) Litres per minute (L/min)

An air dryer in a modern air brake system is installed to:

Increase air reservoir volume Remove moisture and some contaminants from compressed air before it reaches the reservoirs Reduce compressor output Control ABS wheel speed signals

Moisture in air brake systems is a concern because it can:

Improve brake lining life Freeze in lines and valves, causing restricted air flow and malfunctions Increase available brake pressure Reduce corrosion inside reservoirs

An air brake system check valve is used to:

Allow air flow in both directions equally Allow air to flow in one direction but prevent loss of air from a charged reservoir if supply pressure drops Vent air to atmosphere whenever pressure rises Control brake chamber stroke

Dual (split) service air brake systems are used primarily to:

Reduce the number of reservoirs required Provide separate circuits so a failure in one still leaves partial braking available Allow operation of hydraulic brakes Eliminate the need for a parking brake

Foundation brakes in a typical heavy truck drum brake system include:

Wheel cylinders and rotors Brake chambers, pushrods, slack adjusters, S-cams, shoes and drums Springs, struts and shock absorbers Wheel bearings and seals only

When a driver depresses the service brake pedal in a standard air brake system, the treadle (foot) valve:

Mechanically forces the shoes against the drum Meters air from the reservoirs to the service brake chambers in proportion to pedal effort Releases spring brakes only Controls only the trailer emergency line

Brake chamber size is identified by:

Spring colour A number (for example Type 30) related to diaphragm effective area Compressor displacement Reservoir volume

A Type 30 service chamber is commonly used on:

Motorcycles Heavy truck drive axle service brakes Passenger car front brakes Steering systems

Slack adjusters in drum brake systems are used to:

Control reservoir pressure Convert linear pushrod motion to rotary S-cam motion and set shoe-to-drum clearance Dry compressed air Operate the ABS modulator

Automatic slack adjusters must be:

Manually backed off daily Installed and maintained correctly so they can self-adjust with full brake applications Adjusted only with vice grips on the pushrod Adjusted only when the wheels are removed

Excessive brake chamber pushrod stroke usually indicates:

Correct brake adjustment Out-of-adjustment slack adjusters or worn foundation components Low air dryer performance only Incorrect governor cut-out pressure

Brake fade in air drum brakes is most often caused by:

Low air pressure Overheating of linings and drums from prolonged or severe braking High humidity Using dual circuits

The ILM Air Brakes Fundamentals module expects parts technicians to:

Set up and adjust entire brake systems Recognize key components, functions, ratings and service parts used in air brake systems Perform road tests and brake performance certification Program on-board computers

A safety valve in an air system is installed to:

Prevent low pressure conditions Protect reservoirs and components from pressure above a safe maximum Control air dryer purge timing Operate the parking brakes

Spring brake chambers combine:

Only a service chamber with two diaphragms A powerful mechanical spring for parking/emergency plus a service brake diaphragm Two powerful springs for service braking only An air bag and a shock absorber

Caging bolts on spring brake chambers are used to:

Increase spring force during parking Mechanically compress and disable the spring for service or towing when air is not available Adjust slack adjusters automatically Bleed air from the reservoirs

Low air warning devices (light and buzzer) must activate:

Only when parking brakes are applied Before system pressure drops below a specified minimum operating pressure Only when reservoirs are full Any time brakes are applied

Brake linings in air drum brakes are attached to:

S-cam bushings Brake shoes, using rivets or bonding Wheel bearings Slack adjusters

Glazed or oil-contaminated brake linings can:

Improve braking efficiency Reduce friction and lead to pulling or grabbing brakes Have no effect on braking Eliminate brake fade

Brake drums are usually manufactured from:

Aluminum foil Cast iron or specialized alloys designed for heat capacity and strength Plastic composites Copper sheet

Cracked or severely heat-checked brake drums should be:

Reused indefinitely Replaced according to manufacturer and safety guidelines Painted and reinstalled Machined beyond maximum diameter limits

ABS (Antilock Brake Systems) on air-braked vehicles are designed to:

Reduce service pressure permanently Prevent wheel lock-up during hard braking by modulating air pressure Replace the foundation brake system Operate only at low speeds in the yard

With ABS present, correct foundation brake adjustment is:

Less important because ABS controls everything Still essential to ensure proper braking and prevent modulation problems No longer required Only required on trailers

In air brake terminology, 'service brakes' are the:

Parking brakes applied by springs Brakes applied by air pressure from the foot (treadle) valve during normal driving Hand-controlled trailer brakes only Emergency brakes applied by gravity

Parking brakes on heavy vehicles with spring brake chambers are normally held released by:

Mechanical levers Air pressure in the spring brake section Hydraulic pressure Electric motors

Overuse of service brakes on long downhill grades can lead to:

Cooler brakes and better control Overheated drums and linings, brake fade and possible failure Improved ABS operation Longer lining life

On a typical combination vehicle, the total air brake system includes:

Only the tractor front axle brakes Tractor, drive axles, trailer or semitrailer axles and any dollies equipped with air brakes Only the trailer brakes Hydraulic rear brakes and air front brakes

Parts technicians supplying air brake components must ensure parts:

Match thread size only Meet appropriate DOT/transport regulations and manufacturer specifications Are the lowest cost available regardless of rating Fit physically even if ratings differ

Excessive air system leakage may show up as:

Very infrequent compressor cycling Frequent compressor cycling and slow pressure build-up No change in pressure behaviour Lower brake chamber stroke

The supply (feed) line from the compressor to the first reservoir commonly includes:

Only copper tubing A discharge line, sometimes a discharge muffler and an air dryer Only flexible hose with no fittings The service brake chambers

A quick-release valve in a service brake circuit is used to:

Increase reservoir capacity Speed up release of brakes by exhausting air near the chambers instead of back through the foot valve Increase air pressure to the chambers Control parking brake application