An emergency vent valve including a piston responsive to the differential pressure between the brake pipe and a control volume to open a vent valve to vent the brake pipe to atmosphere and includes an exhaust valve responsive to the differential pressure between brake pipe and control volume to connect the control volume to atmosphere through a service check and when the vent valve is opened through a second blow-down choke of a lesser rate which limits the exhausting of the control chamber. A choke valve connected to the piston connects the blow-down choke in series with the service choke when the vent valve becomes fully open. 1871
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