To provide an improved hydraulic brake system and a hydraulic pressure increasing device 10 to be used therein.

The hydraulic pressure increasing device comprises a cylindrical chamber 42. The cylindrical chamber has an inlet end 44 and an outlet end 46. The chamber 42 is sealed at the inlet end 44 by a first end wall 116. An piston assembly 52 is slidably supported in the chamber 42. The piston assembly 52 has a first piston 54 and the first piston has a first outer ring wall 100. An energizing means 60 extending through the chamber 42 in the axial direction energizes the first piston 54 against the first end wall 116. One of the first piston 54 and the first end wall 116 defines an opening portion 110 through which fluid can flow from the inlet end 44 of the chamber 42 into the piston assembly 52.