Leaky Tap

I have a problem with a mixer tap, the cold tap does not close properly, it spins past the closed point and opens again, could this be fixed with a washer or do I need to replace the whole tap unit?
Thanks in advance
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It depends on the style of tap. I take it the tap is a half turn on/off design ? If it is, then it depends on how the innards work. The tap may have had a positive stop key moulded into the body, so if it is now passing that point, then the key has been worn down and is allowing the piston the travel passed and this is causing it to go back to the open position again.
The other designs inside these styles of tap are a ball with a hole through the middle, that when turned to where the holes match up with the opening in the spout outlet the water flows. Or it could be a geared thread which lifts a piston over a certain distance and allows the water to flow. And the other design is a quartered circle, like a chunk out of a pie, which, when turned, allows water to flow passed the open section and out the spout.
You will need to turn the water off and get the head off the tap to see which style yours is before a proper diagnosis can be made.
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Thanks BigWallop, but I think the mixer tap bit may have thrown you off track a bit, for all intents and purposes the actual tap control looks and operates in exactly the same way as a conventional sink tap, it turns on and off in the same way, but it's as though the tap has been forced past its natural closed position.
I do want to change the unit eventually but the one I want to change it for is expensive, and I would need to change the plug hole to match (is this an easy job?), I hoped I could do that after Christmas and just get away with just replacing a washer for now.
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might need to replace jumper as well
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