Commercial Driver Handbook ( PDF ) - California Department of ...

*Air Compressor Governor Cut-In Pressure
Test
To perform this test, the air pressure for the vehicle
cannot be rising when the engine is running. With
the engine idling, slowly pump the brake pedal
to reduce the air tank pressure. Watch the air
pressure gauge between pumps to identify when
the compressor cuts in (needle starts to rise). This
should occur no lower than 85 psi.
For testing purposes, identify where the air
governor cuts in the compressor and verbalize the
minimum pressure at which this can occur.
*Low Air Pressure Warning Device Test
This test may be performed with engine on or off.
To perform the test with the engine off, turn the
electrical power on and have enough air pressure
to keep the low air pressure warning device from
coming on. Slowly pump the brake pedal to reduce
air tank pressure. The low air pressure warning
device must activate between 55 and 75 psi. For
testing purposes, identify when the warning signal
activates, and verbalize the legal range in which
the signal must activate.
If the warning signal does not work, you could
lose air pressure and not know it. This could cause
sudden emergency braking in a single circuit air
system. In dual systems the stopping distance will
be increased. Only limited braking can be done
before the spring brakes come on.
*Check that the spring brakes come on automatically.
Chock the wheels. Release all parking
brakes and shut the engine off. Pump the brake
pedal to reduce the air tank pressure. The trailer
air supply valve knob and tractor protection
valve should pop out when the air pressure falls
to the manufacturer’s specifications (usually in
a range between 20 to 45 psi). This causes the
spring brakes to engage. Some trailers use an air
applied emergency brake system and some trailers
use spring brakes as the emergency brake system.
Check rate of air pressure buildup. With the
engine at operating rpms, the pressure should build
from 85 to 100 psi within 45 seconds in dual air
systems. If the vehicle has larger than minimum
air tanks, the buildup time can be longer and still
be safe. Check the manufacturer’s specifications.
In single air systems (pre-1975), typical requirements
are pressure buildup from 50 to 90 psi
within 3 minutes with the engine at an idle speed
of 600-900 rpms.
If air pressure does not build up fast enough, the
pressure may drop too low during driving, requiring
an emergency stop.
Test service brakes. Wait for normal air pressure,
release the parking brake, move the vehicle forward
slowly (about 5 mph), and apply the brakes firmly
using the brake pedal. Any pulling to one side,
unusual feel, or delayed stopping action should
be checked.
Test parking brake. Fasten your seat belt. Set the
parking brake and try to move the vehicle or allow
the vehicle to slowly move forward and apply the
parking brake. The parking brake should stop a
rolling vehicle, or not allow any movement.
AntIlOck BRAke SyStemS
trailers required to have aBs
• All trailers and converter dollies built on or
after March 1, 1998, are required to have ABS.
However, many trailers and converter dollies
built before this date have been voluntarily
equipped with ABS.
• Trailers will have yellow ABS malfunction
lamps on the left side, either on the front or
rear corner. Converter dollies manufactured on
or after March 1, 1998, are required to have a
lamp on the left side.
• In the case of vehicles manufactured before
the required date, it may be difficult to tell if
the unit is equipped with ABS. Look under the
vehicle for the ECU and wheel speed sensor
wires coming from the back of the brakes.
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