Apparatus in vehicles with compressed air brake systems for actuating the brake system on the detection, by a sensor, of an obstacle in the path of the vehicle

A vehicle fluid brake system with separate service and parking braking valves, comprising in combination, a service brake subsystem having at least one service brake cylinder coupled with a first fluid circuit providing selectively fluid at high pressure by operation of said service braking valve, a parking brake subsystem having at least one parking brake cylinder having a spring loaded piston therein tending to hold the brakes applied, said parking brake subsystem being coupled with a separate fluid circuit providing selectively fluid at high pressure to the piston to overcome the spring force and release the brakes in the absence of operation of said parking braking valve and to permit spring braking to overcome fluid pressure when the parking braking valve is operated, and a further valve assembly having a valve actuated by a force external to the fluid pressures in said fluid circuits connected to operate said service brake subsystem by coupling fluid under pressure directly from said separate fluid circuit to the service brake cylinder to operate the service brake cylinder of the service brake subsystem by said high pressure fluid, and by decoupling the first fluid circuit to prevent loss of said high pressure fluid therein.