Posted 23 January 2013 - 09:30 AM
Don't just change the knuckle cup, as the ball will have been damaged, and will rapidly chew up a new cup. The complete knuckle is not all that expensive, and if one side has failed, the other side is not far behind, so you should do them both, regardless of whatever else, such as Hi-Los, that you are fitting.
Check the knuckle seating in the arm carefully, as if the cup has failed and it has been driven much in that state, the ball may have chewed up the arm, preventing the new cup seating properly. Then it gets expensive, so it really is best to change the knuckles every 40k miles or so, before the cups wear through. Once you have the cone compressor it is fairly simple.
There have been cases of very severe neglect in the 1960s and 1970s, where the ball ground its way so far into the arm that it jammed and sheared off, and again where it ground its way right through, sometimes causing accidents, and always costing 10 times as much as maintaining the knuckles properly. I don't think that any Minis are treated that badly nowadays!