Figure 7-58.Stoplight switch.
stoplight circuit when the brakes are applied. When air
pressure from the treadle valve enters the cavity on the
one side of the diaphragm, the diaphragm changes
position. This action overcomes the force of the spring
and moves the contact plunger until the contacts close.
This closes the stoplight electrical circuit causing the
brake lights to come on. The switch is designed to close
as soon as 5 psi is delivered to it. This means that the
stoplight circuit closes immediately on brake
SERVICING AIR BRAKES
Servicing is the most important part of air brake
maintenance. If the air brake system is kept clean.
tight, and moisture-free, brake failures will be few and
far between. Particular care must be taken to keep the
air compressor intake filters clean and foreign material
out of the lines.
The basic test made to an air brake system is the
operational test. This test may be performed on the
road or in the shop. During an operational test, the
brakes are applied and released while observing for
equal application, sluggish engagement or release,
binding linkage, and exhaust of units.
To check the leakage of the overall system, fully
charge the system. shut off the ignition, and observe
the pressure drop on the gauge mounted on the vehicle
dash. The maximum leakage will be expressed in
pounds per a specific time.
Before making any leakage or pressure
test. consult the manufacturers specifications
for correct pressure and maximum leakage.
To determine if leakage of various components is
within permissible or authorized limits, use the
soapsuds test. To make this test, use a thick mixture of
soapsuds: do not use lye soap. This misture is applied
to places in the system where leakage may occur.
While some places are authorized some amount of
leakage, others are not. For example, castings and the
tube in the governor should have no leakage. Points
with authorized leakage will have a specified
maximum in pounds per a specified time.
Soapsuds can also be used to check the internal
condition of a component. By covering exhaust ports
or casting openings, you can check the condition of the
diaphragms and valves. For example, to check the
condition of the treadle valve, release the brakes and
cover the exhaust ports with soapsuds. Engage the
brakes: any leakage indicates the valve is not sealing
properly. If the diaphragm in the brake chamber is
faulty, leakage will appear around the pushrod with the
As with the drum brake system the linings used
with air brakes gradually wear from use and require
periodic adjustment or replacement. Always consult
the manufacturers specifications before making any
adjustments to the air brake system. This is to ensure
that the correct adjustment is made and that any
variations in procedure are followed.
REVIEW 2 QUESTIONS
What are the three common methods for driving
the air compressor from the engine?
What is the purpose of the governor in an air
What component controls the compressor
On a type O-1 air compressor the pressure range
may be adjusted by adding or removing shims
from what location?
What valve controls the air pressure delivered to
the brake chambers?
What is the function of the relay emergency
What component is used to prevent dirt and
moisture entering unused air lines?
Learning Objective: Describe the operation, terms,
and component functions of an air-over-hydraulic brake