ENGINEER'S BRAKE VALVES 107 



C-7 Brake Valve. The B-12 brake valve was not satisfactory 

 in its operation, so another form, called C-7, was put in service 

 in 1887, in which the main-reservoir air passed through the 

 rotary valve and its seat into the chamber connected to the 

 brake-valve reservoir, and thence through openings in the 

 equalizing piston, as in the B-12. The slide valve and springs 

 in the B-12 were not used in this one, which made it much less 

 complicated and more certain in its action. The perforated 

 equalizing piston gave trouble, as any dirt or foreign substance 

 between the lower side of this piston and the spring washer would 

 allow the air to pass either way through the piston, and the 

 piston would not respond to moderate variations of pressure. 

 To avoid the leakage from the train-pipe exhaust elbow, it 

 was provided with a stop-cock. This could be closed if the 

 piston failed to seat the discharge valve; the cock was usually 

 found closed, and the brake valve used in the emergency posi- 

 tion to discharge air from the train pipe. Cross-sectional views 

 of this valve are shown in Fig. 4 (a) and (6). 



D-8 Brake Valve. The D-8 brake valve, shown in Fig. 5 (a) 

 and (b), was invented in 1889, and was intended to overcome 

 the objectionable features in the valve preceding it. In this valve 

 the main-reservoir air does riot pass through the chamber above 

 the equalizing piston on its way to the train pipe, but [has 

 a direct connection with the train pipe in full-release position 

 through a port and cavities in the rotary valve and seat, and 

 the direct-application-and-supply port that leads to the train 

 pipe. It will be noted that the direct-application-and-supply 

 port in this valve is not a supply port in the valve preceding it, 

 that port being used only in applying the brakes. The bottom 

 of the equalizing-piston stem forms a seat over the train-pipe 

 exhaust port and has a small projection continuing on below 

 the seat to regulate the rate of the train-pipe discharge; the 

 top and bottom faces of the equalizing piston are directly con- 

 nected in full-release and running positions by a small port 

 known as the equalizing port. The valve also has an excess- 

 pressure valve similar to the one in the C-7 valve. In running 

 position, the air from the main reservoir passes this valve on 

 its way to the train ojpe. 



