with the 4 bolt box there are 2 types bushed and roller bearing ,you can tell by postion of locking grub screw for rack adjuster ,slotted screw on engine side of box , bearings been roller dont really wear but can collapse (needles drop out and damage shaft )in which case youd realy need another box ,but you can buy them from bearing suppliers ,one of the main issues is corrosion and pitting were seal sits ,sleeves can be bought to cure this but offhand i dont know were now ,the other issues can be just wear in sector shaft and input usually noticeable by the fact the top adjuster is wound in too far and box still has play ,box needs replacing really as parts are too costly and lastly input shaft ,its 2 shafts in one and the torsion between the 2 gives you the pas steering if thats poor again a new box ,but a reseal and set up can transform a reasonable used box , you dont need any special tools just a uj to fit on input to feel preload etc as its rebuilt