Lesson 3/Learning Event 2
If the brake valve pressure above the diaphragm is reduced, the brake
chamber pressure below the diaphragm lifts the diaphragm. This opens
the exhaust port under the outer edge of the diaphragm, and the
pressure in the brake chamber is exhausted until a lower balanced
pressure is reached. If the air pressure delivered by the brake
valve drops to zero, the relay valve releases all pressure from its
brake chamber. This releases the brakes and the valve returns to its
A relay emergency valve is used in the airbrake system of trailers.
This valve acts as a relay during operation of the brakes. It also
automatically applies the trailer brakes when the air lines to the
towing vehicle are disconnected or broken. The operation of the
relay portion of the valve is much the same as the truck relay valve
during normal operation. However, what happens if the air line is
disconnected or if the trailer breaks away from the towing vehicle is
The relay emergency valve is made to go into its emergency position
and apply the trailer brakes when there is a quick drop in pressure
in the cavity below the emergency diaphragm. For example, if the
emergency line is broken, air pressure in the cavity below the
emergency diaphragm would flow out of the broken emergency line.
Air pressure above the emergency diaphragm will push the diaphragm
down and pull the upper emergency valve down, closing it. Air
pressure above the check valve will hold it down and closed.
Air pressure from the trailer reservoir can now flow across the top
of the depressed emergency diaphragm into the cavity leading to the
brake chambers applying the brakes.
As air pressure escapes from below the emergency diaphragm, pressure
above the pressure regulating diaphragm drops instantly. The spring
below the pressure regulating diaphragm pushes it up, thus, closing
is prevented from escaping through the broken line by the closed
upper emergency valve.
To release the trailer brakes, the air reservoir will have to be
drained of air or the emergency line reconnected to the towing