pe connects the
application cylinder exhaust port in the distributing valve with the
independent brake valve, and through it, when in running position, to
the automatic brake valve. The brake cylinder pipe connects the
distributing valve with the different brake cylinders on the locomotive.
The brake pipe branch pipe connects the distributing valve with the
brake pipe.

127. Q. Explain the operation of the distributing valve when making an
automatic service application of the brake.

A. When the brakes are fully charged, the brake pipe and pressure
chamber pressures are equal, and when a gradual reduction of brake pipe
pressure is made it will be felt in chamber "p" at the right of the
equalizing piston 26, creating a difference in pressure on the two sides
of the piston, causing it to move to the right. The first movement of
the piston closes the feed groove "v", also moves the graduating valve
28, uncovering the service port "z" in the equalizing slide valve 31;
this movement of the piston also causes the shoulder on the end of its
stem to engage the equalizing slide valve, and the continued movement
of the piston moves the valve to service position, in which port "z"
connects with port "h" in the seat of the valve, as shown in Fig. 9. As
the equalizing slide valve chamber is at all times connected to the
pressure chamber, air can now flow from this chamber to both the
application cylinder and chamber through ports "z" and "h", cavity "n"
and port "w" until the pressure on the left or pressure chamber side of
the equalizing piston 26 becomes slightly less than that in the brake
pipe, when the piston and graduating valve will move to the left until
the shoulder on the piston stem strikes the slide valve; this movement
of the graduating valve closes the service port "z", thus closing the
communication between the pressure chamber and application chamber and
cylinder, also closing port "l" which leads to the safety valve. The
distributing valve is now said to be in service lap position. (See Fig.
10.)

128. Q. Upon what does the pressure in the application chamber and
cylinder depend when making a service application of the brake?

A. On the amount of brake pipe reduction; and as the relative volume of
the pressure chamber and application cylinder and chamber is practically
the same as that of an auxiliary reservoir and brake cylinder, it will
be understood that one pound from the pressure chamber will make two and