Air Brakes System Review

1. SYSTEM REVIEW
AIR BRAKES

2. AIR BRAKES REVIEW
 Air Compressor –
Pumps all air for the braking system.
 Air Compressor Governor –
Controls when air is pumped into the
storage tanks. Cut in 100psi; cut out
125psi
 Storage Tanks –
Used to hold compressed air until
needed.
 Air Tank Drains –
Tanks must be drained daily!
 Alcohol Evaporator –
Reduce the risk of ice in winter…
Check & fill daily
 Safety Relief Valve –
First tank - opens at 150 psi.
 Foundation Brakes –
Located at each wheel, most common
type is S-CAM drum brake.
 Supply Pressure Gauge –
ALL trucks & Buses must have; tells how
much air is in the storage tanks.
 Application Pressure Gauge –
NOT on all trucks; shows how much air
you are applying to the brakes.
 Stop Light Switch –
Turns on brake lights;
Electric switch operated by air

3. AIR BRAKES REVIEW
 Low Air Pressure Warning
Device –
Light and/or buzzer comes on before
55 psi to warn of air pressure loss.
 Spring Brakes –
Brakes are held in place when parked
by springs. When truck is moving,
springs are held back by air to allow
wheel to turn. Spring brakes
automatically come on when air
pressure drops below 40 – 20 psi.
 Parking Brake Controls –
Yellow knob on dash for tractor;
Red knob on dash for trailer.
(In Released-Out Applied)
 Modulating Control Valve
(Hand Valve) –
Used to control spring brakes in case the
service brake fails.
NEVER use it for parking.
 Dual Parking Control Valve –
Used in an emergency to move vehicle a
short distance, only has enough air to be
used a few times.
 Antilock Braking System-
Computerized system to control wheel
lock up when braking.
Lamp mounted on left side of trailer;
Light on dash of trucks.

4. AIR BRAKES REVIEW
 Dual Air Brake System –
Two separate air brake systems, one
operates brakes on rear axle (axles),
the other operates the brakes on the
front axle, and possibly one rear axle.
 Service Line - comes loose
enroute, nothing will happen until
you apply the brake pedal
 Emergency Line - comes loose
enroute, air pressure drops to 60 psi
– tractor protection valve closes,
trailer brakes come on
 Air Pressure Build-up –
Check supply pressure gauge, should
build between 85 – 100 psi within 45
seconds.
 Brake Drums –
No cracks or containments (oil/grease).
 Brake Lining –
No less than ¼ thick; No grease/oil
 Check Slack Adjusters -
1. Park on level ground
2. Chock wheels
3. Release brakes
4. Pull on slack adjuster
5. Push rod should not move
more than 1 inch

5. AIR BRAKES REVIEW
 Air Leakage Rates –
 Combination Vehicle
Brakes Engaged = 4 psi in 1 minute
Brakes Released = 3 psi in 1 minute
 Straight Truck or Bus
Brakes Engaged = 3 psi in 1 minute
Brakes Released = 2 psi in 1 minute
 Yellow Bus (School)
Brakes Engaged = 2 psi in 1 minute
Brakes Released = 1 psi in 1 minute
 Normal Operating Pressure
120 – 140 psi
 Test Parking Brakes –
1. Parking brake applied
Yellow knob out;
Red knob in;
Gently pull against tractor.
2. Trailer Brake applied
Red knob out;
Yellow knob in;
Gently pull against trailer.
 Test Service Brake –
Truck in low gear, pull forward at 5 mph,
apply service brake firmly, check for
pulling to one side, unusual feel, or
delayed stopping action.

6. AIR BRAKES REVIEW
 Set up truck for Air Brake
Tests
Place transmission in gear
Shut off engine
Release Brakes
 Low Pressure Warning
Signal –
Light or buzzer comes on by 60 psi.
 Spring Brakes Come On
Automatically –
Fan service brake pedal to lower air
pressure. Tractor protection valve
should close (pop out) at 40 psi;
Parking brake valve should close (pop
out) at 20 psi.
 Controlled Braking –
Apply brakes as hard as you can
WITHOUT locking the wheels start
turning , lock wheels again.
 Stab Braking –
Apply brakes all the way, when wheels
lock up, release brakes, when wheels
 Brake Lag –
The amount of time it takes for air to flow
through air lines to brake chamber.
(Added stopping distance is 32 feet)
 Brake Fading or Failure-
Drums and brake linings getting too hot
and expanding; Having to apply more
pressure to get the same results; Brakes
out of adjustment can also affect this