It should only leak when youve turned the machine off and it is cooling down and when you first turn the machine on.
There is an internal washer/o-ring on many of the valves Ive seen that is replaceable. On some machines replacement of the entire valve is the only option once they start to leak. Im sure Pedro at Coffee Parts could tell you exactly what youll need for your machine if its one of the rebuildable ones. :)
As a temp fix you can try tapping the post that sticks up out of the valve which will release a spurt of steam (Careful!) and cause the valve to pop close under pressure which can then get it to seal completely. Try tapping it several times waiting to see if it seals after each one. Dont mistake the condensed water on the valve boiling off for a leak. If it hasnt sealed after a handful of tries it probably wont.
Java "About to rebuild his" phile