Lesson 2/Learning Event 2
When the truck driver applies the brakes, controlled air from the
truck flows through the service line to the control line inlet at the
top of the emergency relay valve. An internal passage carries the
controlled pressure to the cavity above the relay valve diaphragm.
Because this cavity is small, the controlled air fills it quickly and
moves the diaphragm and relay valve plunger down, compressing the
valve and then compresses the valve spring, opening the air inlet
valve. Compressed air now flows from the trailer reservoir through
the reservoir line opening, past the open air inlet valve, and out
the outlet opening to the brake chamber. The brake chamber rod
extends working the master cylinder and applying the brakes.
When the driver releases the brakes, controlled line pressure is
released from the top of the relay valve diaphragm and the relay
valve plunger. The diaphragm spring pushes the relay valve plunger
up, opening the exhaust valve and permitting the valve spring to
close the air inlet valve. Brake applied air pressure in the brake
chamber is released to the open air by passing through the open
center of the relay valve plunger to a cavity that is connected to
the exhaust port of the emergency relay valve.
The emergency relay valve will maintain an air applied pressure in
the brake chamber that corresponds with the air controlled pressure
from the truck. This is known as the balanced or holding position
and occurs when brake control pressure from the truck pushing down on
the relay diaphragm is equal to the upward pressure on the diaphragm.
Upward pressure is exerted by the diaphragm spring and brake applied
air pressure. The applied pressure is admitted to the bottom side of
the relay diaphragm through an internal passage from the air applied
passage. When pressures on both sides of the diaphragm are equal,
the diaphragm is held in the mid position so that both the air inlet
valve and the air exhaust valve are closed, which maintains a
definite amount of brake applied air pressure. If controlled
pressure is increased, it pushes the valve plunger down opening the
air inlet valve to increase applied pressure. If controlled pressure
is decreased, the valve plunger moves up exhausting some applied
pressure.
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