We repair / replace air brakes components on big trucks in Little Rock,
Three (3) Air Brake Supply systems on Trucks es control your Air Brakes.
The Primary - Controls Rear Brakes, Park and Emergency System
Secondary - Controls Front Brakes.
The Supply System - controls pressure,
supply and regulations of system.
Click air brake diagram picture to enlarge.
All vehicles with air brakes must have a way of stopping if the service brake
system fails. Most vehicle manufacturers combine this
emergency braking system
with a parking-brake system using spring brakes. Spring brakes are not air
applied like service brakes.
They apply when air pressure leaves the brake
chamber and release when air pressure builds up in the chamber.
To check your Air brake system for Air loss, with the engine off and firm pressure
on pedal, you should not see a drop of more than 3 psi in one minute on
REMEMBER: You rarely run out of brakes, but you run out of
The brake components could all be new but if the adjustment is
not done, the brakes will not do their job.
Foundation brakes are the most common air-brake systems found in trucks
es. Using the triple-valve principle, air builds up inside the brake
pipes or air lines, releasing the brakes. Virtually all of the road
going vehicles equipped with air brakes have a graduated release system where a
partial increase in pressure dictates a proportional release in brakes.
Air Brakes Check - Pre Trip
The following components are found in a Foundation Air-Brake System
in commercial trucks or buses:
Air Brake Leak Down Test
- Part of pre-trip inspection and CDL question.
Air Compressor: Pumps the air into storage tanks to be used in the brake system
Air Compressor Governor: Controls the cut-in and cut-out point of the air compressor to maintain a set amount of air in the tank or tanks
Reservoir Tanks: Hold compressed or pressurized air to be used by the braking system
Release valves in the air tanks used to drain the air when the vehicle isn't in
Air Compressor Unloader Valve.
Alcohol Evaporator For Air Brakes
Low Air Pressure Warning
Devices: A low air-pressure warning signal is required on vehicles
with air brakes.
Air Brake Chambers: Cylindrical container that houses a slack adjuster that moves a
diaphragm or cam mechanism.
Adjusters: An arm connects the push rod to the brake s-cam to adjust
the distance between the brake shoes
Push Rod: A steel rod similar to a piston that connects the brake chamber to the slack adjuster.
the brakes are released. If extended, the brakes are applied.
Brake S-Cam: An s-shaped cam
in an air brake system that pushes brake shoes against the brake drum
/ Blocks or Pads: Steel mechanism with a lining that causes friction against the brake drum
Springs: A stiff
spring connected to each of the brake shoes that returns the shoes to the open position when not spread by the s-cam or diaphragm. At idle (foot off the brake
and vehicle's air system charged), air pressure overcomes the diaphragm or the s-cam is in the closed position, resulting in a released brake system. As soon as you depress the brake pedal, the air pressure decreases,
turning the s-cam and spreading the brake shoes against the drum.
pressure drops to between 20 and 45 psi, the vehicles spring brakes will
The compressor refills the reservoir tanks and when you allow the pedal
to retract, the air pressure increases back to the original state. Emergency air brakes complement standard air-brake systems and can be activated by pulling a
button on the dash (near the one with the light that we saw in the introduction).
Before you can drive a vehicle with air brakes, you must push in the
emergency brake button to fill the system with air. As long as the emergency system is pressurized, the emergency brake will remain free. If the system has a
leak, the pressure can decrease enough to engage the emergency brake.
In addition, heavy trucks are often equipped with an exhaust brake that
aids the braking process, but this relies on the engine, not the air-brake system.
WHAT'S THAT SOUND?
Have you ever heard trucks makeing those funny squeaking and hissing sounds?
The squeaking is the air escaping after
braking and the "hiss" air release sound is the automatic bypass safety valves at work, ensuring the air pressure remains at the correct level.
Since a main
advantage of air-brakes systems is their ability to use air to operate, the compressor is constantly kicking on
an off to refill the reservoirs
with pressurized air.
When the compressor builds too much air, the safety valves open, producing that loud hiss.