Figure 8-46. Quick service position.
to the position shown in figure 8-47, called Full Service.
valve 7 to move to the right until the shoulder on the
piston stem strikes the left-hand end of slide valve 3.
In this position, quick-service port y is closed, so that no
As the friction of piston and graduating valve is much
air flows from the brake pipe to the brake cylinder.
less than that of the slide valve, the difference in
Thus, when the brake pipe reduction is sufficiently rapid,
pressure which will move the piston and graduating
there is no need for the additional quick-service
valve, will not be sufficient to move all three;
reduction, and the triple valve automatically cuts out this
consequently, the piston stops in the position shown in
feature of the valve when it is not required. Also, in full-
service position ports z and r are fully open, allowing the
valve to close z, thus cutting off any further flow of air
auxiliary reservoir pressure to reduce more rapidly, so
from the auxiliary reservoir to the brake cylinder and
as to keep pace with the more rapid brake pipe
also port o, thus preventing further flow of air from the
Consequently, no further change in air pressures can
e. Lap Position. When the brake pipe reduction
occur, and this position is called Lap, because all ports
ceases, air continues to flow from the auxiliary reservoir
through ports z and r to the brake cylinder, until the
pressure in the chamber R becomes enough less than
that of the brake pipe to cause piston 4 and graduating