1. In a hydraulic booster having a cylinder and a movable power wall dividing said cylinder into a hydraulic fluid inlet chamber and a hydraulic fluid outlet chamber, said power wall having passage means therethrough including an axially extending recess and connecting said inlet chamber and said outlet chamber: open center valve means for controlling hydraulic fluid flow from said inlet chamber to said outlet chamber through said passage means and including an annular valve seat mounted in said recess for limited radial movement and held against axial movement relative to said power wall, a valve positioned in mating relation with said valve seat and axially movable relative thereto to control hydraulic fluid flow from said inlet chamber to said outlet chamber and having an axial extension extending through said valve seat and provided with an axially extending cylinder-like recess opening into said power wall recess, and first yieldable means between said valve and said valve seat urging them axially apart; a hydraulic reaction piston reciprocably mounted in said power wall recess and in a cylindrical portion of said valve seat and including axially spaced first and second sealed pivotal lands respectively sealed and pivoted on a wall of said power wall recess and said valve seat cylindrical portion and having axially extending passage means therethrough positioned radially inwardly of said lands and providing free fluid communication between the orifice formed by said valve and valve seat and the portion of said power wall passage means communicating with said outlet chamber, said first sealed pivotal land having a smaller diameter than said second sealed pivotal land, said sealed pivotal lands permitting limited radial movement of said valve seat while accommodating free axial movement of said reaction piston relative to said valve seat and said power wall, the wall of said power wall recess and a portion of said reaction piston intermediate and including said lands defining a reaction chamber having a passage in continuous fluid communication with said inlet chamber; second yieldable means between said reaction piston and said valve seat providing line pressure support for said reaction piston; and said valve extension cylinder-like recess having a resilient snubber therein and said reaction piston having a snubber piston thereon extending into said cylinder-like recess in circumferentially spaced relation thereto and movable toward said snubber upon relative closing movement of said reaction piston and said valve to initially cause hydraulic damping of said movement by displacement of hydraulic fluid from said cylinder-like recess past said snubber piston, said snubber piston then being engageable with said snubber to further damp said movement and to transmit reaction force from said reaction piston to said valve.