HYDRAULIC SYSTEMS

An aircraft brake system (210) comprises a supply line for supplying hydraulic fluid to wheel brakes (216), and an associated return line (238). Hydraulic fluid is replenished in at least a portion of the return line both (a) before applying a braking force to the one or more wheels of the aircraft during landing and (b) before an intervening, optional, step if so performed of testing the brake system before landing of the aircraft. Replenishing the hydraulic fluid may be conducted by a control unit (214) automatically and/or in response to a pressure drop in the return line measure by a transducer (244).