PURPOSE: To prevent a normal brake from being affected by air leakage, by using a valve, which acts in response to the pressure difference between a right and a left brake lines, to immediately stop supplying compressed air to the air-leaking line when air starts leaking from one of the brake lines.

CONSTITUTION: When a brake valve 12 is operated, compressed air in an air tank 11 is supplied to brake chambers 17W20 through valves to apply pressure to these chambers. If there is no air leaking from each brake line, no pressure difference is made between the ports 43, 45 of air leak prevention valves 15, 16 so that shuttles 47 are maintained in neutral position and the port 43, 45 are open. If air leaks from one of a right and a left front brake lines, for example, a pressure difference is made between the ports 43, 45 of the valve 15 so that the shuttle 47 comes into tight contact with the pressure-reduced port and closes it. For that reason, the compressed air is not supplied to the air leakage line.