If this is a standard 15mm stop tap with a tee handle it is probably leaking from the gland, simply open the valve fully then turn the hexagonal nut between the valve body and handle until the leak stops. If this is not sucessfull then unscrew the gland nut wind a couple of turns of ptfe tape around the stem and re tighten the gland nut.
If the visolation valve is a quarter turn service valve it will need to be replaced
Chris Nelson Ops manager DD plumbing
June 2009
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