AIR BRAKES - GENERAL INFORMATION
GENERAL INFORMATION
Air brake equipment on trucks and truck tractors
provides a means of controlling brakes through a
medium of compressed air. The air system consists of
various components required to maintain a supply of air,
direct the flow of air and convert energy of compressed
air into mechanical force to apply the brakes. Different
types and sizes of devices are used on different types of
vehicles to meet operating requirements.
The National Highway Traffic Safety Administration
(NHTSA), branch of the Department of Transportation
(DOT), issued a special Federal Motor Vehicle Safety
Standard 121 (FMVSS 121) which specifies that all
chassis equipped with air brakes must meet certain
brake performance requirements.
Some
factors
that
govern
braking
effort
and
effectiveness are brake design, brake lining area, brake
dimensions such as diameter and width, contact
pressure
and
type
of
lining
material
(functional
characteristics). The law has special requirements for
emergency performance which means if air loss occurs
on any one of a vehicle’s axles, the chassis must
demonstrate stopping capability by activation of the
service brake control. All chassis with air brakes will
have dual or split air control circuits consisting of a
primary and secondary system.
AIR SYSTEM
The air system is a split system consisting of a primary
service system and secondary service system.
This split system begins where the main air supply
reservoir branches off into two separate (primary and
secondary) systems. This takes place through a series
of check valves, separate air reservoirs and a brake
valve which has two supply and delivery systems for
service and emergency braking. The purpose of this
split system is to provide a means of making a controlled
stop if a failure occurs in either the primary or secondary
air system. A tractor has the advantage of utilizing the
trailer air brake system during emergency stopping.
There are three reservoirs in the split air system:
Supply Air Reservoir (Wet Tank)
Filled directly by the air compressor. Some air reservoirs
will be a two or three compartment type.
Primary Air Reservoir
Air supply for the major portion of the split service brake
system.
Secondary Air Reservoir
Air supply for the balance of the split system. This
reservoir is also the air pressure supply for the brake
inversion system on straight trucks.
INITIAL SPLIT AIR SYSTEM
Initial FMVSS 121 split air system has the primary
service brake system on all chassis at the rear axle
brakes and secondary service brake system at the front
axle brakes. When the vehicle has tandem rear axles,
the service brakes on both rear axles are in the primary
service brake system. These are termed the "initial" split
air systems. The initial split air system has relay valves
(Figure 1) in the primary system and some park brake
systems. Refer to Figures 3 through 7 for system
identification. You will note that some initial systems
utilize a quick release valve at the front axle in the
secondary system.
Figure 1 - Relay Valve
Figure 2 - Quick Release Valve
CTS-4154Y - Page 2
