Abstract: Disclosed is an air-applied, mechanically-held brake assembly and a vehicle air brake system employing the same. The brake assembly applies and releases the brake upon the supply and exhaust, respectively, of air from a first pressure chamber. The brake assembly locks and unlocks the brake in an applied position by the exhaust and supply, respectively, of air from a second pressure chamber. The locking mechanism of the brake assembly is designed to compound the force of the fluid pressure in the first pressure chamber with the force of the fluid pressure supplied to the second pressure chamber when unlocking the brake to compensate for back pressure.