ENGINEER'S BRAKE VALVES 



105 



emergency position. There was an excess-pressure valve in 

 the rotary valve which, in running position, would retain an 

 excess of pressure in the main reservoir over that in the train 

 pipe. This higher main-reservoir pressure was found necessary 

 to release promptly all triple valves in a train equipped with 

 the automatic brake. 



B-12 Brake Valve. The spring equalization against air 

 pressure was not found satisfactory, so the next valve, known 

 as the B-12 valve, had 

 an equalizing piston 

 with air pressure on 

 both sides of it; this 

 valve is shown in Fig. 

 3. The B-12 brake 

 valve was invented in 

 1886. With this valve 

 the first attempt was 

 made to overcome the 

 evil effects resulting 

 from the engineer clo- 

 sing the exhaust port 

 too suddenly, which on 

 a long train generally 

 .caused several of the 

 head brakes to release. 

 This releasing of the 

 brakes was due to the 

 air (rushing through the 

 train pipe) not stopping 

 as soon as the exhaust 

 port was closed; conse- 



FIG. 3 



quently, the air banked up in the front end of the train 

 pipe and raised the pressure above that in the auxiliary 

 reservoir, thus causing the brakes to release. The equali- 

 zing feature provides for a constant flow of air from the 

 train pipe until the pressure is uniform throughout the train. 

 The valve is also provided with a direct-application port, 

 which is used in cases of emergency only. The chamber 

 above the equalizing piston has a small reservoir attached 



