My 1/4 shut off valve still allows a very small amount of water through when turned off. This valve controls a line that I need to disconnect while long term repairs are in effect and it effectively leaks about a gallon a day. I need complete shut off. What do I do to fix this valve (replacement would be a pain since it's in a tight spot).
Get a pipe cap.
This is normal with shut-off valves that are not exercised for many years.
You only really need it to slow down flow enough to change and tighten the connecting piping. Sometimes that's all you can hope for. That said, we've been on a campaign to replace every one in the facility, but it's almost futile - we'll have the same problem again in 30 years.
"1/4 turn ball valves have plastic seals at each side of the ball with a hole in it. Many times the valve can be tightened (not the stem) but the end that threads into the body to tighten them up and stop the leak. Not all are built this way but most metal ones are. when you removed the fitting it may have loosened causing the leak."
I agreed with this. Either the chrome plated ball had pitted and leak or the plastic or brass cup has scratch. If it's too hard to replace the whole, then buy a same unit and exchange only the new gut.