a deceleration - sensitive braking pressure control unit having a ball valve freely movable in an inlet chamber of the housing of the unit . the ball opens or closes a channel for the passage of pressure fluid through a longitudinally slidable , pressurized stepped piston upon attainment of the change - over pressure with the vehicle loaded . the stepped piston is movable in the longitudinal bore of a pressurized annular piston which is supported slidably in the housing . in this arrangement , the annular piston is slidable in the housing to control the pressure fluid passage between its annular end face and the surface of the ball upon attainment of the change - over pressure , with the vehicle unloaded . both the stepped piston , which contains the channel and the annular piston are comprised of varying effective surfaces acted upon by pressure fluid and are maintained in their initial positions in the housing by springs .