An anti-skid hydraulic braking system incorporates a modulator assembly (3) for modulating the supply of fluid from a master cylinder (2) to a brake (1) in accordance with skid signals. The modulator assembly (3) comprises a de-boost piston (9) located in a bore (8) in a housing (7), an expander chamber (48) connected to the brake (1), a first valve (14) between the master cylinder (2) and the chamber (48), and a second valve (28) for controlling the application to the de-boost piston (9) of a fluid sustained-support force. The modulator assembly (3) incorporates a safety piston (42) which prevents the first valve (14) from closing unless means, suitably an hydraulic accumulator (4) or pump (5), and which provides the fluid sustained-support force are operative.