there is a packing nut that can be tightened on most valves, it looks like a nut under the handle, but don't over tighten! just snug, sometimes this works
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You can also repack the filling under the packing nut. The packing comes as a long string with teflon or graphite pressed onto it. You just take a piece to wind around under the nut. It compresses forming a "washer" that goes around the stem.
See if this gives you any ideas.https://www.instructables.com/id/FIX-A-LEAKY-FAUCET-1/