the pressure on the diaphragm 14 overcomes the tension of the regulating spring 17, when the diaphragm will move to the right, allowing the spring 13 to force the regulating valve 12 to its seat, closing port “K”.  Chambers “G” and “H” are then no longer open to chamber “L” and the feed valve pipe, and these chambers being small, the pressure raises quickly to main reservoir pressure due to the leakage of air past the supply piston 8, which forms but a loose fit in its bushing.  When the pressure in chamber “G” becomes nearly equal to that in chamber “B”, the piston spring “6” forces the piston 8 and its slide valve 9 to closed position, which prevents further flow of air from the main reservoir to the feed valve pipe (see Fig. 6).  The feed valve will remain in closed position until the pressure in chamber “L” is slightly reduced so that the pressure on the diaphragm 14 is no longer able to withstand the pressure of the regulating spring 17, which then forces the diaphragm to the left, lifting the regulating valve 12 from its seat and again opening port “K” to chamber “L”, thus dropping the pressure at the left of piston 8 below that of the main reservoir acting on the opposite side of the piston.

[Illustration:  Fig. 7.  Diagram of B-6 Feed Valve, Open.]

Main reservoir pressure then forces the supply piston and valve over into open position, as shown in Fig. 7, and allows a further flow of air through port “c” to the feed valve pipe to again raise its pressure to the adjustment of the feed valve, when the valve will again close.

102.  Q. What is the duty of the feed valve?

A. To control and maintain a constant pressure in the brake pipe when the brake valve is in running or holding position.

103.  Q. What defect in the feed valve will cause the brake pipe pressure to equalize with that in the main reservoir?

A. This may be caused by a defective feed valve case gasket, permitting main reservoir air to leak into the feed valve pipe, or leakage past the supply valve, or the regulating valve held from its seat, or the supply valve piston too tight a fit in its cylinder.

104.  Q. If the brake pipe charges too slowly when nearing the maximum pressure, where is the trouble?

A. This may be caused by a loose-fitting supply valve piston 8, or the port past the regulating valve 12 partly stopped up.

105.  Q. How should the feed valve be tested?

A. With the brakes released, and charged to the adjustment of the feed valve, create a brake pipe leak of from seven to ten pounds and note the black hand on the brake pipe gauge.  The fluctuation of this hand will indicate the opening and closing of the feed valve, which should not permit a variation of over two pounds in brake pipe pressure; if it does, it indicates a dirty condition of the valve, and should be cleaned.

106.  Q. If the main reservoir pipe connection to the feed valve breaks, what should be done?