I have repaired several similar valves by first loosening the stem packing nut, screw the valve part all the way out (with nut too), then remove the old packing from inside nut, and replace with one or two small appropriately sized O-rings, reassemble, and snug the packing nut just enough to put some resistance on the valve and stop the leak, but not so tight you can't move the valve.
Grigg
