Abstract: A hydraulic booster brake mechanism for use in an open center power steering system of a motor vehicle in which the booster brake mechanism employs a pressure regulating mechanism which limits the maximum pressure at which the booster operates to protect the hydraulic brake system and which operates to make the maximum pressure available when the pressure decreases. The pressure regulating mechanism operates to limit and to make available the maximum pressure when the pressure increases or decreases from a predetermined value by minimizing the effects of frictional forces and dynamic fluid flow by utilizing a control plunger which moves between open and closed positions on fluid trapped in a plurality of grooves which also act as seals. The fluid utilized by the booster is supplied to and delivered from additional grooves which not only assist the sealing grooves but minimize the effect of the flowing fluid.