Brake Systems

Loss of air pressure in the primary air system causes the rear service brakes to become inoperative. The front brakes will continue to be operated by the secondary air system.

Loss of air pressure in the secondary air system causes the front axle brakes to become inoperative. The rear service brakes will continue to be operated by the primary air system.

When air pressure is lost in one air system, the air compressor will operate, but the air supply in the leaking system will not be replenished. There will be enough air in the working system to stop the vehicle safely. The dual air brake system thus provides emergency braking capability. When the low air pressure warning light and emergency buzzer first come on, stop the vehicle immediately. Do not drive the vehicle until the cause of the problem is corrected.

Pulling out the yellow diamond-shaped knob (parking brake control valve) on the auxiliary dash panel applies the parking brakes (spring brakes). See Fig. 12.1 for the standard parking brake control valve and Fig. 12.2 for the Intellipark™ parking brake control valve.

The service brakes and parking brakes have a safety interlock that prevents the unintentional release of the parking brake. These instructions need to be followed for the appropriately equipped vehicle to release the parking brake interlock.

To ensure safe operation and minimum brake wear, follow the steps below when operating the brakes.

To move a vehicle with insufficient system air pressure, it is necessary to release the parking brake springs. Do this by caging (manually releasing) the parking brakes.

After correcting the brake system problem, uncage the parking brakes before resuming normal vehicle operation.

The Bosch hydraulic pin slide disc brake is a two-piston sliding caliper brake and is used at both the front and rear wheel locations. Each pin slide caliper disc brake wheel installation is made up of a caliper assembly, anchor plate assembly, and disc brake pads.

Before driving the vehicle, secure all loose items in the bus so that they will not fly forward during a full brake application. Check that the brake system warning light is off after releasing the hand brake. If the warning light does not go off, correct the problem before continuing operation of the vehicle.

During normal braking stops, depress the brake pedal until braking action slows down the vehicle. Increase or decrease the pressure on the pedal so that the vehicle comes to a smooth, safe stop. Apply the hand parking brake if the vehicle is to be parked.

When the brake pedal is depressed, compressed air enters the brake chamber, causing the diaphragm to move a pushrod assembly.

The pushrod turns the slack adjuster and brake camshaft. As the camshaft turns, the S-type cam head forces the brake shoes against the brake drum and braking occurs.

When the brakes are released and air is exhausted from the brake chamber, the actuator return spring (within the brake chamber) and the brake shoe return spring return the camshaft, brake shoes, slack adjuster, and pushrod to their released positions.

The Meritor WABCO ABS is a four-sensor system. It combines one front-axle control channel with one rear-axle control channel to form one control circuit.

Example: The sensor and solenoid control valve at the left front wheel form a control circuit with the sensor and solenoid valve at the right rear axle.

ABS includes signal-generating tone wheels and sensors located in the wheel hubs of each sensed axle. The sensors transmit vehicle wheel speed information to an electronic control unit.

During emergency or reduced traction stops, fully depress the brake pedal until the vehicle comes to a safe stop. Do not pump the brake pedal. With the brake pedal fully depressed, the ABS will control all wheels to provide steering control and a reduced braking distance.

Although the ABS improves vehicle control during emergency braking situations, the driver still has the responsibility to change driving styles depending on the existing traffic and road conditions. For example, the ABS cannot prevent an accident if the driver is speeding or following too closely on slippery road surfaces.

Even if the ABS is partially or completely inoperative, normal braking ability is usually maintained.

Automatic slack adjusters should never be manually adjusted except during routine maintenance of the foundation brakes (e.g., replacing shoes), during slack adjuster installation, or in an emergency situation.

When the brake pushrod stroke exceeds the legal brake adjustment limit on a vehicle, there is likely a mechanical problem with the foundation brake components or the adjuster is improperly installed.

Visit a repair facility as soon as possible when brakes equipped with automatic slack adjusters are determined to be out of adjustment.

Vehicles with electronic engines and ABS may have Automatic Traction Control (ATC). On these vehicles, the ATC system automatically limits wheel spin during reduced-traction situations. In normal braking applications, the standard air brake system is in effect.

During reduced-traction situations, the ATC solenoid valve controls air pressure to the modulator valves and they in turn increase, hold, or reduce pressure to the appropriate brake chambers to provide better traction whenever wheel spin occurs.

When the ATC system is in the NORMAL mode, it will apply gentle braking to the spinning wheel, to improve power to the wheel(s) with better traction. If both wheels are spinning, the system will signal the engine to reduce power.

ATC may include a deep snow and mud option to increase available traction on extra soft surfaces like snow, mud, or gravel. If so equipped, a momentary contact rocker switch labeled ATC will be located on the dash. Pressing the switch will temporarily allow more wheel spin. The activation of the deep snow and mud option is indicated by a flashing WHEEL SPIN lamp. Pressing the switch again will cycle the system back to normal operation.

After the ignition switch is turned on, the ABS indicator light and the WHEEL SPIN indicator light illuminate for about three seconds. After three seconds, the warning lights go out if all of the ABS components are working.

During vehicle operation, solid illumination of the ABS light indicates a problem with the vehicle ABS. Repair the ABS immediately to ensure full braking capability.

If equipped with ATC, an amber WHEEL SPIN indicator light illuminates if one of the drive wheels spins during acceleration. When the light illuminates, partially release the throttle pedal to stop the spinning wheel. The light goes out when the wheel stops spinning.

If slippery road conditions continue, turn on the differential lock switch. See Chapter 11 for axle switch instructions.

Electronic Stability Control (ESC) automatically reduces engine power, applies the engine brake (if so equipped), and/or applies the brakes when the acceleration sensor detects that the vehicle is at risk of rolling over. In addition, ESC offers the added capability of complete directional stability (yaw control) in oversteer and understeer conditions to reduce the likelihood of drift-out. The system determines where the driver is attempting to steer the vehicle and how much brake demand is required in order to more precisely control the vehicle in an emergency situation.

ESC works by constantly comparing the driver’s intention with the vehicle's actual behavior. The system does this by monitoring systems such as wheel speed, steering angle, yaw rate, lateral acceleration, throttle position, and brake application. A central microcomputer analyzes the collected data and triggers a response to keep the vehicle on course when an unstable condition is detected.

The roll stability control system automatically reduces engine power, applies the engine brake (if so equipped), and/or applies the brakes when the acceleration sensor detects that the vehicle is at risk of rolling over. The control can intervene even before an advisory message is displayed.

When the system detects that the vehicle is at risk of oversteering or understeering, it applies individual wheel end brakes, activates the engine brake (if so equipped), and/or cuts engine power, depending on the severity. As a result, the driver has full control over the vehicle until the system detects a potential risk and intervenes accordingly. ESC operates automatically; the driver does not monitor or activate the system. When ESC activates, an indicator lamp illuminates on the instrument panel. See Fig. 12.5 for the indicator lamp used on the ICU3 instrument panel and Fig. 12.6 for the indicator lamp used on the Ametek instrument panel.

An exhaust brake is an optional auxiliary braking system that assists, but does not replace, the service brake system. The driver can use the exhaust brake, in combination with the accelerator and clutch pedal (if so equipped), to make maximum use of the exhaust brake in off-highway and mountain driving as well as in traffic or high-speed highway driving.

The exhaust brake is controlled by a dash-mounted rocker switch, shown in Fig. 12.7 , to help slow the vehicle when the accelerator is released.

To turn the exhaust brake on, press on the upper part of the rocker switch. When the exhaust brake switch is on, an amber light emitting diode (LED) illuminates inside the switch. When the panel lights are on, the EXHST BRK legend is backlit in green.

The exhaust brake is only active when engine speed is between 1100 and 2700 rpm. Depressing the accelerator deactivates the exhaust brake. The ABS system, when active, also deactivates the exhaust brake.

The exhaust brake is a butterfly valve mounted in the exhaust pipe. When the driver's foot is not on the accelerator pedal and the upper half of the exhaust brake switch is pressed in, with the amber light on the switch illuminated, an air cylinder shuts the butterfly valve, which restricts the flow of exhaust gases and retards the engine. This retarding action is carried through the engine and drivetrain, slowing the vehicle and reducing the need for frequent service brake applications.

Exhaust brakes are not intended for use as the primary braking system during vehicle operation.

Before starting the engine, make sure that the lower half of the exhaust brake switch is pressed in and the amber light is not illuminated. Do not turn the exhaust brake on until the engine has reached normal operating temperatures.

When you remove your foot from the accelerator, and the upper half of the exhaust brake switch is pressed in with the amber LED illuminated, the exhaust brake is applied. The following conditions should exist if the brake is operating properly:

While approaching a steep grade, make sure that the upper half of the exhaust brake switch is pressed in, with the amber LED illuminated. The exhaust brake comes on as soon as you remove your foot from the accelerator pedal. While going down the grade, use a low enough gear to safely descend with a minimum application of the service brakes. As a general guideline, use the same gear as you would to ascend the hill.

Apply the service brakes to reduce the engine rpm or make a slower descent by using a lower gear.

Make sure the exhaust brake switch is turned off before shutting down the engine.

The following information provides steps to perform an air brake test for vehicles equipped with the Bendix Intellipark park brake system. The park brake switch has two light emitting diodes (LEDs). See Fig. 12.2 . Both LEDs illuminate when the parking brake is set; the LEDs are extinguished when the parking brake is released.

For electric powered vehicles, turn the ignition key to the ON position. This keeps the high-voltage system from operating. To start the high-voltage system, turn the ignition key to the start position and release.