Blowback or blowdown is caused by pushrods that have the consistancy of wet noodles and flex or bend under compression load. Any bends at all will also contributre to this. To have a rock solid link if a single push/pull type must be perfectly straight from servo arm to to surface horn and any fuselage exit holes must be in the appropriate location Which often on many arfs they are not and you are expected to bend the rod ends a little assuring that your airplane will have some blowdown.