Lesson 3/Learning Event 2
As soon as the pressure in the brake chamber and connecting lines
equals the brake valve pressure above the diaphragm, the force of the
spring below the diaphragm forces the outer edge of the diaphragm
back up against the body. At the same time, the center of the
diaphragm keeps the exhaust port closed. This is the holding
If the brake valve pressure on top of the diaphragm is reduced, the
brake chamber pressure raises the diaphragm. This opens the exhaust
port and lets the brake chamber pressure escape through the exhaust
port. If the brake valve pressure on top of the diaphragm is only
partially released, the diaphragm goes to the holding position as
soon as the pressure above and below it is equal.
As stated earlier, a relay valve is used on the rear wheels of trucks
to speed up the application and release of the rear brakes.
When the valve is in the released position, the reservoir pressure is
in the cavity below the supply valve which is closed. The diaphragm
is in its normal position resting on the diaphragm guide.
When the brake valve is applied, it sends air through the brake valve
port into the cavity above the diaphragm. The pressure pushes the
diaphragm down. When the diaphragm moves down, its outer edge seals
the exhaust port and the center of the diaphragm forces the diaphragm
guide and the supply valve down. This opens the supply valve and
allows air pressure from the reservoir to flow through the supply
valve and into the cavity below the diaphragm. This cavity is
connected through the brake chamber port to the brake chambers. With
the mechanism in this position, air pressure is flowing directly from
the reservoir through the relay valve into the brake chambers
applying the brakes.
As soon as the air pressure below the diaphragm equals the air
pressure above the diaphragm, the force of the supply valve spring
lifts the center of the diaphragm and closes the supply valve. This
limits the air pressure being delivered to the brake chambers by the
relay valve to the same pressure as that being delivered by the brake
valve to the relay valve. In this position, the supply valve is
closed and the force of air pressure on top of the diaphragm keeps
the outer edge of the diaphragm down sealing the exhaust port. An
increase in brake valve pressure repeats the action (as in the
applying position) until the two pressures are again equal.