Why is my steam wand leaking

Mine is leaking water rather than steam. But maybe a water leak comes from the same root cause? I determined that the plastic handle has worn and the steam control lever now moves beyond the valve's fully closed position to a position which is partially open. So if you back up the lever a few degrees it might stop the flow.

I have yet to figure out if there is a way to replace the plastic lever/handle piece.

Same issue. It was fine then started dripping water very very slowly, and now it's a regular spitting of steam - not a lot but it's annoying.

Replace the steam valve mechanism for the wand with a new one. Some people have suggested that you can just clean the mechanism and reverse the seals but whilst this may work for a while, the ball in the mechanism gets pitted and rough and will never seal like a new ball. The mechanism is around $60 Aust. The hardest part of the job is probably getting the plastic cover off to get to the screw to take the handle off. I suggest cutting two 10mm strips from a credit card. Then insert these in the gap between the handle and the plastic cover. You will need a sharp blade to spread the plastic a little the get the credit card in on BOTH sides of the handle. Once you do this the plastic cover will pull off easy. There is enough room between the plastic and body of the machine (spreading the plastic apart about 0.8mm each side to make it slip of easy. You will also need an O ring kit as you will need to replace red O rings. For those that have leaking/dripping out of the descaling ports, many suggest to tighten these screws. These screws do not seem to operate thru tightening. Mine stopped leaking when I actually unscrewed it out one or two turns. The screw slot was about horizontal (I dont know if that is important or not). Just know that you may have to play with the screw by slowly undoing it to stop the leaking. (my left screw is the full way in, the right one out a couple of turns to stop the leaking)