Abstract: A vehicle braking system incorporating two circuits serving driven and non-driven axles respectively and in which both air-actuated service brake units and spring brake units for parking are provided on both axles. The system includes relay valving which is operable upon failure of the service braking in either circuit and responsive to the service pressure in the other circuit to provide a graduated actuation of the spring brake units in the circuit where service braking has failed. The spring brake on the driven axle and/or the relay valving is so constructed as to attenuate the force applied by the spring brake units on the driven axle.

Abstract: A fluid-pressure operated dual circuit braking system in which each circuit serves a rear axle brake and at least one front axle brake, one circuit only including a load sensing valve. The braking pressure in the other circuit is controlled by a relay valve in response to the modulated pressure from the load sensing valve. The relay valve includes a piston which is normally balanced but moves in the event of a failure in the circuit embodying the load sensing valve, to connect the unmodulated pressure in the relay valve circuit directly to the rear axle brakes.