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 one-half pounds in the application chamber and cylinder; in other words, with the pressure chamber charged to seventy pounds and no pressure in the application chamber and cylinder, if they were connected and the pressure allowed to equalize it would do so at about fifty pounds; that is, twenty pounds from the pressure chamber will make fifty pounds in the application chamber and cylinder.

[Illustration:  Fig. 9.  Automatic Service.]

129.  Q. How is the application piston 10 affected by the air pressure in the application cylinder “g”?

A. Pressure forming in this cylinder will force the piston to the right; the piston in moving will carry with it the exhaust valve 16, closing the exhaust ports “e” and “d”, at the same time moving the application valve 5, opening the supply port “b”, allowing main reservoir air from chamber “a” to flow through ports “b” and “C” to the connection marked “CYLS”, and on to the different brake cylinders of the locomotive until the pressure in the brake cylinders and at the right of the application piston becomes slightly greater than that in chamber “g” when the application piston and valve will move back to lap position as shown in Figures 9 and 10.