pe "IV" and on to the automatic brake valve, where the
port to which this pipe leads is blanked by the automatic rotary valve,
which prevents the air leaving the control cylinder and reservoir, thus
holding the locomotive brake applied while the train brakes are being
released. The movement of the parts are the same where the release is
made in holding position.

178. Q. Explain the movements of the parts in the control valve when the
automatic brake valve is moved to running position, after having first
been moved to release or holding position.

[Illustration: Automatic Control Valve. Service Lap Position. Fig. 23.]

A. In this position of the brake valve the port to which the release
pipe "IV" is connected is open to the exhaust, thus allowing the air in
the control cylinder and reservoir to escape to the atmosphere. The
reduction of pressure in the control cylinder "D" below that in chamber
"B" causes the control piston 2 to move up, carrying with it the exhaust
valve 7 to release position, opening the exhaust port "N", thus allowing
the air to return from the brake cylinders through ports "C" and "N" to
the atmosphere, releasing the brake. (See Fig. 21.)

179. Q. Explain what takes place in the control valve when an automatic
emergency application of the brake is made.

A. Any sudden reduction of brake pipe pressure will be felt on the brake
pipe side of piston 3, and will cause it and the valve 4 to move to
their extreme upper position, the knob on the piston striking the
graduating stem 13, causing it to compress the spring 14, moving the
emergency valve 15 upward, opening port "Q"; this allows brake pipe air
to flow against valve 16, unseating it, then through port "T" to the
brake cylinder. (See Fig. 24.) In the meantime auxiliary reservoir air
can flow past the end of the slide valve through port "E" to the control
cylinder "D" and control reservoir, forcing piston 2 downward unseating
valves "I" and "IA", thus allowing main reservoir air to flow to the
brake cylinders, applying the brake.

180. Q. At what pressure will the auxiliary reservoir and control
reservoir equalize when using seventy pounds brake pipe pressure?

A. At about fifty pounds; however, with the automatic brake valve in
emergency position, there is a small port in the rotary valve (called
the blow-down timing port) opened to the control reservoir pipe and
control reservoir which allows main reservoir air to flow to the control
reservo