an apparatus connectable to a brake master cylinder for regulating pressure in a hydraulic brake system . the apparatus includes an outer casing housing a cylinder portion and a piston slidable therewithin . a pressure chamber forms between the cylinder wall and the piston . the piston includes a seal within an outer groove to prevent fluid leakage . the cylinder portion includes an inlet hole for hydraulic fluid flow into the cylinder and an air discharge valve . a fixing member having an extension with a concave portion is threadably attached to the lower end of the case , and a rod portion of the piston slidably fits into the concave portion . a spring providing a constant pre - pressure is located between a shoulder of the piston and a pressure controlling member abutting the fixing member , such that the end of the rod remains a fixed distance from the bottom of the concave portion .