When a brake application is made, air pressure from the brake valve enters the cavity below the diaphragm. The air
pressure below the diaphragm moves the piston until it contacts the leaf spring. The leaf spring travels past a fulcrum at
which point the leaf spring snaps a shorting bar which mates with the contact strips. The stop light electrical circuit is
completed, lighting the stop lights before the brake applications pressure reaches 6 psi (41 kPa/0.41 bar).
Parking and Service Brakes
The top priority of the air system is to provide braking. Each wheel has two spring brake chambers and two service brake
chambers. The spring brake is spring actuated and must be released by pressurized air. The spring brakes on all four
wheels are released by the parking brake push-pull knob on the right hand console in the cab. Pushing down on the
parking brake causes air pressure to enter the spring brake chamber on each wheel and compress the spring, releasing
the brakes. At least 40 psi (275.8 kPa/2.75 bar) is required to keep the parking brake valve engaged. If supply pressure
to the valve drops below 40 psi (275.8 kPa/2.75 bar), the valve will release, applying the brakes.
The service brakes are applied by air pressure. Depressing the foot brake pedal on the cab floor causes pressurized air to
enter the service brake chamber on each wheel, applying the brakes. In the event of a loss of supply pressure to the
service brakes, the spring brake valve will allow the operator to release or bleed off the air pressure in the spring brake
chamber by depressing the foot brake pedal to apply the brakes.
Dual Brake Valve
The dual brake valve is used in the air system when a single function or component must be controlled by either of two
sources of pressure. The dual brake valve transmits the higher of the two pressure sources to the outlet port.
As air pressure enters either end of the dual brake valve (inlet port), the movable shuttle responds to the air pressure and
seals the line on the opposite inlet port, but permits air pressure to flow out the outlet port. The same action will take place
if the air pressure on one side of the shuttle is higher than the air pressure on the other side. It is not necessary for the
cavity, one side of the disc or shuttle, to be exhausted for the valve to operate.
Dual brake valves are designed so that it is impossible for the shuttle to block off the outlet port.