Basic cure for leakage is to replace the lower seal. If there's not too much wear in the lower bush that should keep the oil in for a good bunch of miles. Pitman arm has to come off and a suitable thin but tough, jawed puller, is a bit hard to find. The arm and bottom of the shaft are marked for correct straight ahead replacement. This has to be correct so you know that the box is exactly in its central position when adjusting the top screw. All of the workshop manuals go into this with great detail but assume you are adjusting the box off the car. You're not likely to adjust it with a spring tension gauge but these details allow you to understand why and how to make the adjustments on the car. If the adjustment ends up a fraction loose, no worries, but a fraction tight will wear some more off the worm until it is slack. If tight you'll also get wander when driving in a straight line as the box tries to get off the high spot.