Parking and emergency braking system

Claim 1

A vehicle parking and emergency braking valve which controls a spring brake to be used in a dual hydraulic braking system of a vehicle comprising: a housing with a front and rear compartment; the front compartment having a pressure port, brake control port, and return port, the rear compartment having a first brake pressure port and a second brake pressure port; valve means for controlling flow of pressurized fluid from said pressure port to said brake control port; a slidable shaft with a communicating passage therethrough for allowing pressurized fluid to flow from said brake control port out said return port; during the operation of the vehicle, said slidable shaft is normally positioned so that said valve means blocks the flow of pressurized fluid out said return port, and said pressure port and brake control port are in communication; a control piston means for pushing said slidable shaft, said control piston means being pushed by one of two actuating means, said first actuating means being an external force communicated to said control piston and said second actuating means being a pressure differential between said first and second brake pressure ports communicated to said control piston means; and override means for preventing said second means from communicating said pressure differential to said control piston means unless said pressure differential exceeds a predetermined amount, wherein: said spring brake is applied when the pushing force of said control piston means is removed to allow said slidable shaft to move thereby seating said valve means to stop flow from said pressure port to said brake control port and, subsequently, unblocking said communicating passage to allow pressurized fluid to flow from said brake control port to said return port.