Air Brake System Operation - freightliner BUSINESS CLASS M2 Driver Manual

Air Brake System, General
Information
The warning light and buzzer come on if air pressure
drops below 64 to 76 psi (441 to 524 kPa) in either
system. If this happens, check the dual system air
pressure gauge to determine which system has low
air pressure. Although the vehicle's speed can be
reduced using the brake pedal, either the front or
rear service brakes will not be operating at full ca-
pacity, causing a longer stopping distance. Bring the
vehicle to a safe stop and have the air system re-
paired before continuing.
IMPORTANT: In the event of a total loss of ser-
vice brakes with full system air pressure, use
the parking brake control valve (yellow knob) to
bring the vehicle to a complete stop in the saf-
est location possible.
Before a vehicle with insufficient system air pressure
can be moved, the spring parking brakes must be
released by applying an external air source at the
gladhands, or by manually caging the parking brake
springs.
WARNING
Do not release the spring parking brakes and
then drive the vehicle. There would be no means
of stopping the vehicle, which could result in se-
rious personal injury or vehicle damage. Before
releasing the spring parking brakes, make the
connection to a towing vehicle or chock the tires.
After correcting the brake system problem, uncage
the spring parking brakes before resuming normal
vehicle operation.
Primary Air Brake System
Loss of air pressure in the primary air system causes
the rear service brakes to become inoperative. The
secondary air system will continue to operate the
front brakes and the trailer brakes (if equipped).
Secondary Air System
Loss of air pressure in the secondary air system
causes the front axle brakes to become inoperative.
The primary air system will continue to operate the
rear service brakes and the trailer brakes (if
equipped).
Steering and Brake Systems

Air Brake System Operation

Before driving your vehicle, allow time for the air
compressor to build up a minimum of 100 psi (689
kPa) pressure in both the primary and secondary
systems. Monitor the air pressure system by observ-
ing the dual system air pressure gauge and the low-
air-pressure warning light and buzzer. The warning
light and buzzer shut off when air pressure in both
systems reaches 64 to 76 psi (441 to 524 kPa).
IMPORTANT: Before driving the vehicle, secure
all loose items in the cab so that they will not fly
forward during a full brake application. Make
sure all passengers are wearing seat belts.
During normal brake 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
parking brake if the vehicle is to be parked.
IMPORTANT: An air brake proportioning system
may be used in tractor air brake systems when
the vehicle is not equipped with antilock braking
system (ABS). When operating in bobtail mode,
the rear brake chambers (on which the axle
load has been greatly reduced) receive reduced
(proportional) air pressure, while the front axle
brake chambers receive full (normal) air pres-
sure. This results in a different brake pedal
"feel," as the pedal seems to require more travel
and/or effort to slow or stop the vehicle. How-
ever, the air brake proportioning system actually
improves vehicle control when the tractor is in
bobtail mode. When the tractor is towing a
trailer, the rear brake chambers will receive full
(normal) application air pressure.
When parking a vehicle attached to a trailer that
does not have spring parking brakes, apply the trac-
tor parking brake. Chock the trailer tires before dis-
connecting the vehicle from the trailer.
WARNING
If a trailer or combination vehicle is not equipped
with spring parking brakes, do not park it by pull-
ing out only the trailer air supply valve knob.
This would apply only the trailer service brakes.
If air were to bleed from the trailer brake system,
9.2