Page 177 - Driving Commercial Vehicles Manual+

driving commercial vehicles

This is accomplished by the use of a two-way check valve. The air that is
delivered from the two-way check valve is frequently called blended air.

Blended air

The two-way check valve
draws air from the tank with
the highest pressure.

The two-way check valve has two inlet ports and one delivery port.
A free-floating shuttle within the valve will move away from the inlet that has
the higher pressure, and the higher pressure will be supplied to the parking
brake control.
This arrangement will also ensure that the spring parking brakes will
not automatically apply if there’s a total loss of air pressure in either reservoir.

Dual air brake system — partial system failure

With the ruptured air line
from the primary reservoir,
the driver can still make a
controlled stop.

This diagram shows the benefit of the blended air supply for the parking
brake system. There has been a loss of air from the primary reservoir. The
two-way check valve shuttle has moved so that secondary reservoir pressure
supplies the parking brake control valve.

162