I would recommend checking the ball valve washer as they sometimes can distort and deteriorate especially in the older all brass type valves.
Turn off the water supply, remove the split pin from the valve ,unscrew the end cap and take out the valve plunger, you will see the washer at the end of the plunger which should be flat with no grooves or nicks etc.
If it's damaged, replace it, sometimes for a temporary fix you can reverse them , but a new washer costs a few pence only.By the way, the valve plunger is a 2 part item, put a wide screwdriver in the slot and use pliers or grips to unscrew the washer cap.
Clean the brass parts before reassembly and smear with vaseline, this fix should last you several years.
Pete. (38 years plumbing experience), January 2007
