There is one feed valve for each drivers brake valve fitted between the brake valve and the release pipe. It regulates the train pipe vacuum to 21 inches and maintains this so long as the release pipe vacuum is above 21 inches.
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1 Top Body 2X Valve 3X Sleeve for Valve 5 Valve Seating 6 Adaptor Nut 7 Diaphragm Plate 8 Adaptor 9X Bottom Body 10 Spindle 11 Diaphragm 12 Spring 13 Adjusting Plate 14 Adjusting Nut 15X Bottom Plate 16 Cover 17 Lock Nut 18 Washer 19 Diaphragm 20 Studs and Nuts, 3/8" BSW 21 Bolts and Nuts, 3/8" BSW 22 Shakeproof Locking Washers, 3/8" i/d 23X Joint |
Initially the valve is held off its seat by spring tension and vacuum is allowed to build up in the train pipe. When the vacuum above the main diaphragm is 21 inches atmospheric pressure overcomes spring tension and seats the diaphragm, so opening the valve, air is now extracted by the exhausters and the valve closes again at 21 inches.
The smaller diaphragm is to balance the pressure under the valve.
When changing ends a difference in the train pipe vacuum may be noticed, this means that one of the adjustable feed valves needs resetting.