It's a matter of to much water pressure from the outside.
Many people sell the item, here is the listing from GoWesty
http://www.gowesty.com/Merchant2/merchant.mvc?
Screen=PROD&Product_Code=GVW-25660&Category_Code=cg-h20

Good Luck
On Aug 12, 2006, at 2:25 PM, William Treadway wrote:

> Bought a new (to me) 90 westy syncro
> Running through the systems, I noticed that the sink faucet allowed
> outside water to run regardless of knob position.
> Bought a "new" faucet assy from Bus Depot and installed it. Same
> issue.
> Faucet works fine on internal tankage but when you hook it up to
> external water, the faucet cannot shut it off.
> Here is the really strange part.
> I took the replaced faucet and blew into the feed for external water
> and the valve kept me from blowing  through the faucet when it is in
> the off position. I am sure that I installed the new faucet because
> the wiring harness sheath is much longer on the new one than the one
> I replaced.
> I did notice what MIGHT have been mung on the water threads of the
> replacement faucet.
> Is there something I overlooked or did I get a bad faucet... or am I
> in some parallel universe?
> ______
> Bill Treadway
> Broker Associate
> John Daugherty Realtors
> 281-639-4444  Direct
> 281-850-8141 Cell
> 832-550-2615 Fax