i've just finished installing the new servo and mastercylinder on mine and if i were you i'd do the same as my brakes are so much better the unit on mine came of a tdci mondeo (54 plate) but you can use any unit of focus/transit/mondeo etc of the newer breads only really difference is the position of the hole servo pipe goes in and the mondeo is in the same place thats why i used it the assistance is great just like all the newer stuff, and your problem is down to either the spring that raps around the transfer bar has gone weak or the one hidden in the servo has gone weak