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Operation & Maintenance Instruction
2.2 EMERGENCY - FIRST STAGE
When an emergency rate of brake pipe reduction occurs,
the brake pipe air pressure in cavities b, b1 and b2 is reduced
rapidly creating a differential between the brake pipe pressure
in cavities b, b1 and b2 and the air pressure in the control
pressure volume Q. This causes the control diaphragm and
piston assembly to move, unseating the piston stem from its
seat on the exhaust valve and seating the piston on the piston
seat of the body. The seating of the piston isolates cavity b2
from cavity b1. Air pressure in cavity b2 is then directed to
exhaust through the passage in the unseated piston stem at
the exhaust valve. Cavity b2 is exhausted quickly due to its
small volume.
Figure 2 - Emergency Position - 1st Stage
Diagrammatic View
2.3 EMERGENCY - SECOND STAGE
The rapid exhaust of air pressure from cavity b2 creates a
differential on the non-spring side of the vent valve diaphragm
assembly causing this assembly to move away from its seat
and connects brake pipe air from port 1 and cavities b and b1
to exhaust (Port EX). Air pressure in the control pressure
volume Q then flows to atmosphere through the charging
choke of the control piston, cavity b1, passage B, cavity b
and port EX, when the air pressure in the control pressure
volume Q is vented, the vent valve diaphragm assembly and
the control diaphragm and piston assembly are returned to
the Release Position by the force exerted by the spring.
Figure 3 - Emergency Position - 2nd Stage
Page 3 of 10
Diagrammatic View
September, 2017
4211-7, S.7
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