Abstract: A hydraulic valve actuation system has a hydraulic actuator connected with a valve in an engine. The hydraulic actuator has a boost pin enclosing a drive pin in a casing. The drive pin connects with and defines a valve lash with a valve stem of the valve. The hydraulic actuator applies and releases an actuating force through the drive pin to the valve stem that selectively opens and closes the valve during engine operation. The hydraulic actuator adjusts the valve lash during one or more rest periods. By adjusting the valve lash during the rest periods, the hydraulic actuator essentially adjusts the valve lash in response to one or more of the actual physical parameters of the valve during engine operation.