Go to hardware and purchase a length of faucet stem packing string. Should have a graphite/teflon impregnated string you can wind around the valve stem under the cap before you install the whole thing in the valve body and tighten the cap down.
Such a product may not be commonly available everywhere. I also have raveled it out of pump shaft packing "rope". I usually keep some of that round for a couple irrigation pumps. Would be happy to send you a few inches in an envelope if you need.
You can also use teflon tape wound under there.