I've had the problem with this type of valve, and solved it by doing the
Disassemble the valve by removing the cotter pin, spring, and stop plate from
bottom and pulling upward on the handle. Once the handle is out, you can see
the barrel on the handle that fits into the valve body is tapered, as is the
in the valve body.. Clean everything up, being sure to remove all grit from the
handle barrel and tapered seat in the valve body. smear some brass polish or
rubbing compound on the barrel, re-insert into the valve body. Turn it round
round in the valve body, as though you were "lapping in" cylinder head valves.
This will re-mate the sealing surfaces. Clean off the valve again, re-assemble
and give it a try! Also, if posible, replace the spring on the bottom. They are
usally rusted and very weak.
hope this helps.