Lesson 3/Learning Event 2
When the inlet valve opens, air pressure flows from the reservoir
through the valve to the brake chambers. This applies the brakes.
When the air pressure being delivered to the brake chambers, from the
opening below the diaphragm, overcomes the mechanical force being
applied to the top of the diaphragm, the diaphragm lifts. This
permits the inlet valve to close preventing any further rise of air
pressure in the brake chambers, while the exhaust valve remains
closed and prevents any escape of air pressure.
If the driver pushes the foot pedal farther down, more mechanical
force is applied to the top of the diaphragm. When this happens,
more pressure is delivered to the brake chamber and is applied to the
If the driver lets the pedal move toward its released position, the
force on top of the diaphragm is reduced. Air pressure below the
diaphragm overcomes the mechanical force on top of it, and the
diaphragm lifts slightly. When this happens, the inlet valve remains
closed and the exhaust valve opens. This exhausts air pressure from
the brake chambers until the air pressure below the diaphragm again
balances the mechanical force on top of it.
If the driver lets the foot pedal return to the fully released
position, the exhaust valve remains open. Thus, all the pressure
from the brake chamber is exhausted, and the brakes are fully
If the driver pushes the pedal down to the fully applied position,
the pressure regulating spring is compressed until the spring guide
strikes the spring slat. This holds the rocker arm down, the inlet
valve is held open, and full reservoir pressure is allowed to pass
through the brake valve to the brake chambers.
The trailer brake control valve is used to apply and release the
trailer brakes without applying the brakes of the towing vehicle.
This type valve is usually mounted on the steering column or on the
instrument panel. The driver may put the handle in any one of
several positions between the released and fully applied position.