A hydraulic brake master cylinder includes a cylinder unit having first and second spaces, a channel communicating the spaces and an adjuster exposed outside; a piston slidably disposed in the first space; a first spring applying an elastic force to the piston; a rod moving relative to the piston; a second spring applying an elastic force to the rod; a seal attached to the rod and moving controllably to seal the channel; and a nut threaded onto the rod. The adjuster is rotatable to make the nut and the rod rotate with respect to each other so as to change the distance between the seal and the end of the channel to adjust the free stroke.