Air Brakes Truck and Tractor

On a typical highway tractor, the dual service circuits usually supply:

One circuit for headlights and one for horn One circuit for front (steer) axle brakes and one for drive axle brakes One circuit for tractor and one for trailer lighting Only the trailer brakes

A tractor protection valve is installed to:

Control engine fuel pressure Protect the tractor’s air supply if the trailer or supply line fails Limit maximum tractor speed Control cab air suspension

The trailer supply (emergency) line on a tractor is typically colour coded:

Blue Red Green Yellow

The service (control) line to the trailer is typically colour coded:

Blue Red Yellow Black

Glad hands on the back of the tractor are used to:

Connect electrical power to the trailer Connect tractor and trailer air lines using standard couplers Attach safety chains Adjust fifth wheel height

Dummy (parking) couplers on the back of the cab are used to:

Store unused hoses and protect glad hand seals from dirt and damage Increase air pressure Attach hydraulic hoses Lift the trailer

The tractor parking control valve (dash valve) normally:

Applies only the front axle brakes Controls air to the spring brake sections on tractor axles Directly applies service brakes with hydraulic pressure Controls engine shutdown

The trailer parking (supply) valve in the tractor cab is typically:

A blue hand valve A red, push-pull valve used to charge and exhaust the trailer supply line A floor-mounted pedal An electronic switch on the steering wheel

Pulling out the trailer supply valve in the cab will:

Charge the trailer system with air Exhaust air from the trailer supply line and apply trailer spring brakes Release all service brakes Shut down the engine

The hand (trolley) valve on some tractors is used to:

Apply tractor front brakes only Apply trailer service brakes independently for control or testing Control engine retarder Set axle alignment

Using the hand valve for normal speed control instead of the foot valve is:

Recommended for everyday driving Not recommended, as it can overheat trailer brakes and cause uneven wear Mandatory by regulation Required whenever descending hills

On a dual-circuit tractor, a failure in the front axle service circuit should:

Remove all braking capability Still allow braking on the drive axle circuit, although stopping distance is increased Apply the trailer brakes automatically Release parking brakes

Air reservoirs on a tractor are normally mounted:

Only inside the cab On the frame rails or crossmembers where they are supported and protected On top of the cab Inside the fuel tank

Multi-circuit protection valves are used so that:

All circuits drain together if one fails Each circuit is isolated so a failure in one does not completely empty the others Only the front axle brakes receive air Only the trailer receives air

In many tractors, an application pressure gauge shows:

Reservoir pressure only Service air pressure being delivered to the brake chambers during a brake application Tire pressure Compressor outlet temperature

Tractor ABS typically monitors wheel speed on:

Only the steer axle Selected wheels on steer and drive axles, depending on configuration Trailer axles only The fifth wheel plate

ABS warning lamps on the tractor dashboard indicate that:

Brakes will not operate at all The ABS system has detected a fault and has disabled ABS modulation on affected wheels The parking brake is applied The air dryer is purging

The fifth wheel on a tractor is related to air brakes because it:

Provides an air reservoir mounting location Supports the trailer and provides a pathway for air and electrical lines via hoses and cables Acts as an ABS sensor Controls governor cut-out

Typical foundation brake arrangement on a highway tractor with tandem drives is:

Disc brakes on front axle only Drum or disc brakes on steer and both drive axles, all air-operated Hydraulic front and air rear Electric brakes on all axles

A common tractor air system test involves:

Measuring coolant pressure Checking compressor build-up time, governor cut-in/cut-out and system leakage rates Measuring engine oil pressure only Testing tire tread depth

When a tractor is operated without a trailer (bobtail), the air brake system:

Is disabled Still operates the tractor service and parking brakes using its own reservoirs Cannot be used Must be filled with water

Tractor air tanks are often labelled 'primary' and 'secondary' so that:

Drivers know which tank to fill with water Circuits to specific axles can be identified and diagnosed Only one tank is used for braking Trailer brakes can be adjusted

Parts technicians supplying valves or chambers for tractor air systems must match:

Colour and approximate size only Port sizes, mounting style, function, crack pressures and approvals shown in catalogues Any available part that threads in Only the brand name

A bobtail (tractor only) may have different braking characteristics because:

Weight on drive axles is higher Weight on drive axles is lower and brakes can lock more easily without trailer load There are more brakes applied The compressor is disconnected

The supply line from the tractor to the trailer also functions as:

The trailer service control line The emergency line that charges trailer reservoirs and controls trailer spring brakes A vent line for ABS A cooling line for the compressor

Pressure protection valves on tractors may be used to:

Isolate non-essential air-operated accessories until reservoir pressure reaches a safe level Increase pressure to steering brakes Reduce reservoir size Control fuel delivery

A typical tractor may use an in-line air dryer located:

Between the reservoirs and the brake chambers In the discharge line between the compressor and the first reservoir In the trailer service line only In the cab heater duct

The ILM Air Brakes Truck and Tractor module emphasizes that parts technicians should:

Carry out brake performance tests on the road Recognize tractor air system layouts and identify correct replacement components Adjust automatic slack adjusters Program ABS ECUs

Some tractors have additional reservoirs dedicated to:

Cab air suspension, horns and other accessories Only engine lubrication Fuel tank pressurization Hydraulic steering

Service manuals and ILMs show that typical compressor cut-out pressures for highway tractors are:

Extremely low (for example 200–300 kPa) In a higher range (for example 690–930 kPa or 100–135 psi depending on jurisdiction) Exactly 50 psi on all vehicles Always beyond component ratings

If the tractor protection valve closes due to a serious trailer air leak, the tractor service brakes:

Lose all air supply Continue to function from the tractor’s own reservoirs Release immediately Lock full on

On modern tractors with disc brakes, the air brake system operates:

Mechanical linkages only Air-operated disc brake actuators instead of S-cam drum components Hydraulic calipers only Electric motors at each wheel

Parking a tractor with the transmission in gear but without applying spring brakes:

Is acceptable if on level ground Is unsafe because driveline forces may not hold and air leakage can release service brakes Prevents any movement Is required by regulation

When ordering air tanks for tractors, key information includes:

Only approximate length Capacity, diameter, length, port size and location, mounting style and certification markings Colour and finish only The type of fuel used in the truck

On tractors with central tire inflation or air ride seats, the accessory air system is usually supplied from:

A separate electric compressor only The main air system through pressure protection and check valves Hydraulic pumps Coolant lines

Steer axle brakes on tractors are critical for:

Only holding the truck when parked Directional control and balanced braking during stops Trailer braking only Controlling governor cut-out

A common feature of tractor air systems is a dual-needle gauge that:

Shows oil temperature and coolant level Displays pressures in the primary and secondary service reservoirs Shows trailer tire pressure Measures governor speed

When coupling a tractor to a trailer, charging the trailer air system is done by:

Pulling out the trailer supply valve Pushing in (opening) the trailer supply valve after lines are connected Using only the hand valve Applying the parking brake