Improved quick service limiting valve for freight brake control valve device

摘要

A quick service limiting valve device for a freight brake control valve employing a primary valve in a fluid flow path via which brake pipe air is connected to the brake cylinder during a secondary phase of quick service to propagate the brake pipe reduction signal until the brake cylinder pressure acting on the limiting valve control piston effects closure of the primary valve to terminate the quick service flow of brake pipe air to the brake cylinder when a predetermined brake cylinder limit pressure is realized. A secondary valve is arranged to interrupt the brake pipe to brake cylinder flow path downstream of the primary valve in the event leakage at the primary valve causes the pressure acting on the quick service limiting valve control piston to exceed the predetermined limit pressure. Until this limit pressure is exceeded, the secondary valve is bypassed so that only the primary valve controls the brake pipe to brake cylinder flow path via the limiting valve, thereby reducing hysteresis due to friction to obtain more efficient limiting valve operation.