I think I may need to replace the shutoff valves (angle stops) for a bathroom sink. The current ones do not shut off completely for some reason. In the past, they have sometimes dripped where the rotating shaft feeds through the end cap (not sure of the names). The current ones are multi-turn.
Would quarter turn valves be better in this application? Are there any disadvantages of them?
Ah, leaky valves. Music to a recovering submariner's ears-- from Hitchcock's "Psycho" soundtrack.
That Family Handyman link is great advice, and a crescent wrench works as well as their pliers. We've had a subscription to that magazine for at least five years, and it's saved us literally thousands of dollars in repair bills.
But back to your original question. Those valves get such infrequent use that their o-rings/packing can not only dry out, crack, and start leaking-- but pieces of the o-rings can break off and get into the water stream to further gum up the valve's internal seating surfaces.
Sometimes the simplest solution is to exercise the valve. Turn on the sink faucets and start the water running, then go underneath and give each isolation valve three or four cycles from full open to full shut. If they start to leak around the shaft (stem) going through the end cap (bonnet nut) then tighten the bonnet nut in 1/8th-turn increments until the dripping stops. Check the stem/bonnet after another day or two to make sure that things haven't relaxed and started leaking again.
If after this exercise the valves don't shut tightly and completely stop the water flow, then you could just replace them with the same model of valves (which are relatively cheap) or with the quarter-turn ball valves (which are a tad more expensive). The ball valves have their own issues but will work as reliably as the multi-turn throttle valves.
Submarines have literally thousands of valves of different design/construction, and in its infinite wisdom the Navy has a "preventive maintenance system" and a "small-valve grooming schedule" to lovingly care for each and every one of them. You can imagine the mechanic's glad cries of great joy as they climb all over the engine room's nooks & crannies trying to [-]find[/-] work on valves that are sometimes touched only once or twice a year. The homeowner's version of this philosophy would be to cycle all the isolation valves a couple times a year to find the problems before they find you. With these occasional checkups, even the cheapest isolation valves will last for 20-30 years.