Lesson 2/Learning Event 2
Now let's see how the emergency relay valve provides automatic
breakaway protection. Since the emergency air line is connected to
the air supply inlet opening, air pressure from the truck reservoir
is present on the bottom side of the emergency valve diaphragm. Air
pressure from the trailer reservoir is present on the top side of the
emergency valve diaphragm from the air supply passage and the
reservoir line inlet opening. Air can pass from the truck reservoir
to the trailer reservoir through the oneway check valve in the
emergency valve diaphragm, so that during normal operation the air
pressure above and below the diaphragm is equal. At this time the
emergency valve diaphragm release spring will hold the diaphragm in
the raised position.
Assume that there is a loss of air pressure in the emergency line.
This could be caused by the trailer breaking away from the truck and
breaking the line or by a leaking or disconnected emergency air hose
coupling. This removes the air pressure on the bottom side of the
emergency valve diaphragm. Air pressure from the trailer reservoir
on top of the diaphragm pushes the diaphragm down compressing the
diaphragm release spring. The downward movement is transferred to
the air inlet valve allowing compressed air to flow from the
reservoir to the brake chamber locking the brakes.
The relay valve plunger will remain in the applied position as long
as there is air pressure in the trailer reservoir or until the
service line is reconnected to a truck. To release the trailer
brakes without connecting the trailer to a truck, the drain cock in
the bottom of the reservoir should be opened to allow all the
compressed air in the system to escape.
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