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may have to remind riders about rules, or to keep
arms and heads inside the bus.
4.3.2 – At Stops
Riders can stumble when getting on or off, and
when the bus starts or stops. Caution riders to watch
their step when leaving the bus. Wait for them to sit
down or brace themselves before starting. Starting
and stopping should be as smooth as possible to
avoid rider injury.
Occasionally, you may have a drunk or disruptive
rider. You must ensure this rider's safety as well as
that of others. Don't discharge such riders where it
would be unsafe for them. It may be safer at the next
scheduled stop or a well-lighted area where there
are other people. Many carriers have guidelines for
handling disruptive riders.
4.3.3 – Common Accidents
The Most Common Bus Accidents. Bus accidents
often happen at intersections. Use caution, even if
a signal or stop sign controls other traffic. School
and mass transit buses sometimes scrape off
mirrors or hit passing vehicles when pulling out from
a bus stop. Remember the clearance your bus
needs, and watch for poles and tree limbs at stops.
Know the size of the gap your bus needs to
accelerate and merge with traffic. Wait for the gap
to open before leaving the stop. Never assume
other drivers will brake to give you room when you
signal or start to pull out.
4.3.4 – Speed on Curves
Crashes on curves that kill people and destroy
buses result from excessive speed, often when rain
or snow has made the road slippery. Every banked
curve has a safe "design speed." In good weather,
the posted speed is safe for cars but it may be too
high for many buses. With good traction, the bus
may roll over; with poor traction, it might slide off the
curve. Reduce speed for curves! If your bus leans
toward the outside on a banked curve, you are
driving too fast.
4.3.5 – Railroad-highway Crossing/ Stops
Stop at RR Crossings:
Stop your bus between 15 and 50 feet before
railroad crossings.
Listen and look in both directions for trains. You
should open your forward door if it improves your
ability to see or hear an approaching train.
Before crossing after a train has passed, make sure
there isn't another train coming in the other direction
on other tracks.
If your bus has a manual transmission, never
change gears while crossing the tracks.
You do not have to stop, but must slow down and
carefully check for other vehicles:
At streetcar crossings.
Where a policeman or flagman is directing traffic.
If a traffic signal is green.
At crossings marked as "exempt" or "abandoned."
4.3.6 – Drawbridges
Stop at Drawbridges. Stop at drawbridges that do
not have a signal light or traffic control attendant.
Stop at least 50 feet before the draw of the bridge.
Look to make sure the draw is completely closed
before crossing. You do not need to stop, but must
slow down and make sure it's safe, when:
There is a traffic light showing green.
The bridge has an attendant or traffic officer who
controls traffic whenever the bridge opens.
4.4 – After-trip Vehicle Inspection
Inspect your bus at the end of each shift. If you work
for an interstate carrier, you must complete a written
inspection report for each bus driven. The report
must specify each bus and list any defect that would
affect safety or result in a breakdown. If there are no
defects, the report should say so.
Riders sometimes damage safety-related parts
such as handholds, seats, emergency exits, and
windows. If you report this damage at the end of a
shift, mechanics can make repairs before the bus
goes out again. Mass transit drivers should also
make sure passenger signaling devices and brake-
door interlocks work properly.
4.5 – Prohibited Practices
Avoid fueling your bus with riders on board unless
absolutely necessary. Never refuel in a closed
building with riders on board.
Don't talk with riders, or engage in any other
distracting activity, while driving.
Do not tow or push a disabled bus with riders aboard
the vehicle, unless getting off would be unsafe. Only
tow or push the bus to the nearest safe spot to
discharge passengers. Follow your employer's
guidelines on towing or pushing disabled buses.
Section 4 - Transporting Passengers Safely Page 4-3
Version: July 2017
Commercial Driver’s License Manual – 2005 CDL Testing System
4.6 – Use of Brake-door Interlocks
Urban mass transit coaches may have a brake and
accelerator interlock system. The interlock applies
the brakes and holds the throttle in idle position
when the rear door is open. The interlock releases
when you close the rear door. Do not use this safety
feature in place of the parking brake.
Section 4
Test Your Knowledge
1. Name some things to check in the interior of a
bus during a Vehicle inspection.
2. What are some hazardous materials you can
transport by bus?
3. What are some hazardous materials you can’t
transport by bus?
4. What is a standee line?
5. Does it matter where you make a disruptive
passenger get off the bus?
6. How far from a railroad crossing should you
stop?
7. When must you stop before crossing a
drawbridge?
8. Describe from memory the “prohibited
practices” listed in the manual.
9. The rear door of a transit bus has to be open to
put on the parking brake. True or False?
These questions may be on your test. If you can’t
answer them all, re-read Section 4.
Section 4 - Transporting Passengers Safely Page 4-4
Version: July 2017
Commercial Driver’s License Manual – 2005 CDL Testing System
Section 5
AIR BRAKES
This Section Covers
• Air Brake System Parts
• Dual Air Brake Systems
• Inspecting Air Brakes
• Using Air Brakes
This section tells you about air brakes. If you want
to drive a truck or bus with air brakes, or pull a trailer
with air brakes, you need to read this section. If you
want to pull a trailer with air brakes, you also need
to read Section 6, Combination Vehicles.
Air brakes use compressed air to make the brakes
work. Air brakes are a good and safe way of
stopping large and heavy vehicles, but the brakes
must be well maintained and used properly.
Air brakes are really three different braking systems:
service brake, parking brake, and emergency brake.
The service brake system applies and releases the
brakes when you use the brake pedal during
normal driving.
The parking brake system applies and releases the
parking brakes when you use the parking brake
control.
The emergency brake system uses parts of the
service and parking brake systems to stop the
vehicle in a brake system failure.
The parts of these systems are discussed in greater
detail below.
5.1 – The Parts of an Air Brake System
There are many parts to an air brake system. You
should know about the parts discussed here.
5.1.1 – Air Compressor
The air compressor pumps air into the air storage
tanks (reservoirs). The air compressor is connected
to the engine through gears or a v-belt. The
compressor may be air cooled or may be cooled by
the engine cooling system. It may have its own oil
supply or be lubricated by engine oil. If the
compressor has its own oil supply, check the oil
level before driving.
5.1.2 – Air Compressor Governor
The governor controls when the air compressor will
pump air into the air storage tanks. When air tank
pressure rises to the "cut-out" level (around 125
pounds per-square-inch or "psi"), the governor
stops the compressor from pumping air. When the
tank pressure falls to the "cut-in" pressure (around
100 psi), the governor allows the compressor to
start pumping again.
5.1.3 – Air Storage Tanks
Air storage tanks are used to hold compressed air.
The number and size of air tanks varies among
vehicles. The tanks will hold enough air to allow the
brakes to be used several times, even if the
compressor stops working.
5.1.4 – Air Tank Drains
Compressed air usually has some water and some
compressor oil in it, which is bad for the air brake
system. For example, the water can freeze in cold
weather and cause brake failure. The water and oil
tend to collect in the bottom of the air tank. Be sure
that you drain the air tanks completely. Each air tank
is equipped with a drain valve in the bottom. There
are two types:
Manually operated by turning a quarter turn or by
pulling a cable. You must drain the tanks yourself
at the end of each day of driving. See Figure 5.1.
Automatic--the water and oil are automatically
expelled. These tanks may be equipped for
manual draining as well.
Automatic air tanks are available with electric
heating devices. These help prevent freezing of the
automatic drain in cold weather.
Figure 5.1
Section 5 – Air Brakes Page 5-1
Version: July 2017
Commercial Driver’s License Manual – 2005 CDL Testing System
5.1.5 – Alcohol Evaporator
Some air brake systems have an alcohol evaporator
to put alcohol into the air system. This helps to
reduce the risk of ice in air brake valves and other
parts during cold weather. Ice inside the system can
make the brakes stop working.
Check the alcohol container and fill up as
necessary, every day during cold weather. Daily air
tank drainage is still needed to get rid of water and
oil. (Unless the system has automatic drain valves.)
5.1.6 – Safety Valve
A safety relief valve is installed in the first tank the
air compressor pumps air to. The safety valve
protects the tank and the rest of the system from too
much pressure. The valve is usually set to open at
150 psi. If the safety valve releases air, something
is wrong. Have the fault fixed by a mechanic.
5.1.7 – The Brake Pedal
You put on the brakes by pushing down the brake
pedal. (It is also called the foot valve or treadle
valve.) Pushing the pedal down harder applies more
air pressure. Letting up on the brake pedal reduces
the air pressure and releases the brakes. Releasing
the brakes lets some compressed air go out of the
system, so the air pressure in the tanks is reduced.
It must be made up by the air compressor. Pressing
and releasing the pedal unnecessarily can let air out
faster than the compressor can replace it. If the
pressure gets too low, the brakes won't work.
5.1.8 – Foundation Brakes
Foundation brakes are used at each wheel. The
most common type is the s-cam drum brake. The
parts of the brake are discussed below.
Brake Drums, Shoes, and Linings. Brake drums
are located on each end of the vehicle's axles. The
wheels are bolted to the drums. The braking
mechanism is inside the drum. To stop, the brake
shoes and linings are pushed against the inside of
the drum. This causes friction, which slows the
vehicle (and creates heat). The heat a drum can
take without damage depends on how hard and how
long the brakes are used. Too much heat can make
the brakes stop working.
S-cam Brakes. When you push the brake pedal, air
is let into each brake chamber. Air pressure pushes
the rod out, moving the slack adjuster, thus twisting
the brake camshaft. This turns the s-cam (so called
because it is shaped like the letter "S"). The s-cam
forces the brake shoes away from one another and
presses them against the inside of the brake drum.
When you release the brake pedal, the s-cam
rotates back and a spring pulls the brake shoes
away from the drum, letting the wheels roll freely
again. See Figure 5.2.
Figure 5.2
Wedge Brakes. In this type of brake, the brake
chamber push rod pushes a wedge directly between
the ends of two brake shoes. This shoves them
apart and against the inside of the brake drum.
Wedge brakes may have a single brake chamber,
or two brake chambers, pushing wedges in at both
ends of the brake shoes. Wedge type brakes may
be self-adjusting or may require manual adjustment.
Disc Brakes. In air-operated disc brakes, air
pressure acts on a brake chamber and slack
adjuster, like s-cam brakes. But instead of the s-
cam, a "power screw" is used. The pressure of the
brake chamber on the slack adjuster turns the
power screw. The power screw clamps the disc or
rotor between the brake lining pads of a caliper,
similar to a large c-clamp.
Wedge brakes and disc brakes are less common
than s-cam brakes.
5.1.9 – Supply Pressure Gauges
All vehicles with air brakes have a pressure gauge
connected to the air tank. If the vehicle has a dual
air brake system, there will be a gauge for each half
of the system. (Or a single gauge with two needles.)
Dual systems will be discussed later. These gauges
tell you how much pressure is in the air tanks.
5.1.10 – Application Pressure Gauge
This gauge shows how much air pressure you are
applying to the brakes. (This gauge is not on all
Section 5 – Air Brakes Page 5-2
Version: July 2017
Commercial Driver’s License Manual – 2005 CDL Testing System
vehicles.) Increasing application pressure to hold
the same speed means the brakes are fading. You
should slow down and use a lower gear. The need
for increased pressure can also be caused by
brakes out of adjustment, air leaks, or mechanical
problems.
5.1.11 – Low Air Pressure Warning
A low air pressure warning signal is required on
vehicles with air brakes. A warning signal you can
see must come on before the air pressure in the
tanks falls below 55 psi. (Or one half the compressor
governor cutout pressure on older vehicles.) The
warning is usually a red light. A buzzer may also
come on.
Another type of warning is the "wig wag." This
device drops a mechanical arm into your view when
the pressure in the system drops below 55 psi. An
automatic wig wag will rise out of your view when
the pressure in the system goes above 55 psi. The
manual reset type must be placed in the "out of
view" position manually. It will not stay in place until
the pressure in the system is above 55 psi.
On large buses it is common for the low pressure
warning devices to signal at 80-85 psi.
5.1.12 – Stop Light Switch
Drivers behind you must be warned when you put
your brakes on. The air brake system does this with
an electric switch that works by air pressure. The
switch turns on the brake lights when you put on the
air brakes.
5.1.13 – Front Brake Limiting Valve
Some older vehicles (made before 1975) have a
front brake limiting valve and a control in the cab.
The control is usually marked "normal" and
"slippery." When you put the control in the "slippery"
position, the limiting valve cuts the "normal" air
pressure to the front brakes by half. Limiting valves
were used to reduce the chance of the front wheels
skidding on slippery surfaces. However, they
actually reduce the stopping power of the vehicle.
Front wheel braking is good under all conditions.
Tests have shown front wheel skids from braking
are not likely even on ice. Make sure the control is
in the "normal" position to have normal stopping
power.
Many vehicles have automatic front wheel limiting
valves. They reduce the air to the front brakes
except when the brakes are put on very hard (55 psi
or more application pressure). These valves cannot
be controlled by the driver.
5.1.14 – Spring Brakes
All trucks, truck tractors, and buses must be
equipped with emergency brakes and parking
brakes. They must be held on by mechanical force
(because air pressure can eventually leak away).
Spring brakes are usually used to meet these
needs. When driving, powerful springs are held
back by air pressure. If the air pressure is removed,
the springs put on the brakes. A parking brake
control in the cab allows the driver to let the air out
of the spring brakes. This lets the springs put the
brakes on. A leak in the air brake system, which
causes all the air to be lost, will also cause the
springs to put on the brakes.
Tractor and straight truck spring brakes will come
fully on when air pressure drops to a range of 20 to
45 psi (typically 20 to 30 psi). Do not wait for the
brakes to come on automatically. When the low air
pressure warning light and buzzer first come on,
bring the vehicle to a safe stop right away, while you
can still control the brakes.
The braking power of spring brakes depends on the
brakes being in adjustment. If the brakes are not
adjusted properly, neither the regular brakes nor the
emergency/parking brakes will work right.
5.1.15 – Parking Brake Controls
In newer vehicles with air brakes, you put on the
parking brakes using a diamond-shaped, yellow,
push-pull control knob. You pull the knob out to put
the parking brakes (spring brakes) on, and push it
in to release them. On older vehicles, the parking
brakes may be controlled by a lever. Use the
parking brakes whenever you park.
Caution. Never push the brake pedal down when
the spring brakes are on. If you do, the brakes could
be damaged by the combined forces of the springs
and the air pressure. Many brake systems are
designed so this will not happen. But not all systems
are set up that way, and those that are may not
always work. It is much better to develop the habit
of not pushing the brake pedal down when the
spring brakes are on.
Modulating Control Valves. In some vehicles a
control handle on the dash board may be used to
apply the spring brakes gradually. This is called a
modulating valve. It is spring-loaded so you have a
feel for the braking action. The more you move the
control lever, the harder the spring brakes come on.
They work this way so you can control the spring
brakes if the service brakes fail. When parking a
vehicle with a modulating control valve, move the
Section 5 – Air Brakes Page 5-3
Version: July 2017
Commercial Driver’s License Manual – 2005 CDL Testing System
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