Page 48 - 2021-2022 Commercial Drivers Manual

SECTION 5 : AIR BRAKES

TRACTOR PROTECTION VALVE AIR BRAKE SYSTEM COMPONENTS AND LOCATION

& EMERGENCY TRAILER (SINGLE CIRCUIT SYSTEM)
BRAKE OPERATION

Figure 5.3 Figure 5.4

March 1, 1998 are required to have a lamp on ping distance, but it does help you keep the mary and secondary air pressure gauges (or

the left side. vehicle under control during hard braking. needles, if the system has two needles in one
On newer vehicles, the malfunction lamp gauge). Pay attention to the low air pressure
comes on at start-up for a bulb check, and then 5.2 – Dual Air Brake warning light and buzzer. The warning light
goes out quickly. On older systems, the lamp and buzzer should shut off when air pressure
could stay on until you are driving over five Most heavy-duty vehicles use dual air brake in both systems rises to a value set by the manu-
mph. systems for safety. A dual air brake system has facturer. This value must be greater than 60 psi.
If the lamp stays on after the bulb check, or two separate air brake systems, which use a The warning light and buzzer should come
goes on once you are under way, you may have single set of brake controls. Each system has on before the air pressure drops below 60 psi
lost ABS control at one or more wheels. its own air tanks, hoses, lines, etc. One system in either system. If this happens while driving,
In the case of towed units manufactured typically operates the regular brakes on the you should stop right away and safely park the
before it was required by the Department of rear axle or axles. The other system operates the vehicle. If one air system is very low on pres-
Transportation, it may be difficult to tell if the regular brakes on the front axle (and possibly sure, either the front or the rear brakes will not
unit is equipped with ABS. Look under the ve- one rear axle). Both systems supply air to the be operating fully. This means it will take you
hicle for the electronic control unit (ECU) and trailer (if there is one). The first system is called longer to stop. Bring the vehicle to a safe stop,
wheel speed sensor wires coming from the back the “primary” system. The other is called the and have the air brakes system fixed.
of the brakes. “secondary” system. See Figure 5.4.
ABS is an addition to your normal brakes. Before driving a vehicle with a dual air sys- 5.3 – Inspecting Air Brake Systems
It does not decrease or increase your normal tem, allow time for the air compressor to build You should use the basic seven-step inspec-
braking capability. ABS only activates when up a minimum of 100 psi pressure in both the tion procedure described in Section 2 to inspect
wheels are about to lock up. primary and secondary systems. Watch the pri- your vehicle. There are more things to inspect
ABS does not necessarily shorten your stop-
on a vehicle with air brakes than one without
them. These things are discussed below, in the
order they fit into the seven-step method.
TEST YOUR KNOWLEDGE

5.3.1 – During Step 2 Engine
Compartment Checks
Subsection 5.1
Check Air Compressor Drive Belt (if com-
• Why must air tanks be drained? • What are spring brakes? pressor is belt-driven). If the air compressor is
• What is a supply pressure gauge used • Front wheel brakes are good under all belt-driven, check the condition and tightness
for? conditions. True or False? of the belt. It should be in good condition.
• All vehicles with air brakes must have a • How do you know if your vehicle is 5.3.2 – During Step 5 Walk-around

low air pressure warning signal. True or equipped with antilock brakes? Inspection
False?
Check Slack Adjusters on S-cam Brakes. Park
These questions may be on your test. If you can’t answer them all, re-read subsection 5.1. on level ground and chock the wheels to prevent
the vehicle from moving. Release the parking
brakes so you can move the slack adjusters. Use

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