Shower runs when unwanted

I installed a new tub-shower. The plumbing mostly works.... but: water runs out of the shower head when the one-piece mixer is in the neutral position. If--when wishing to fill the tub and not shower--the mixer handle is turned a little to the 'cold' or 'hot' sides no water flows out of the shower head.
What gives? TIA
David Todtman
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On Tue, 07 Aug 2007 13:42:04 +0000, David Todtman wrote:

What kind/brand of valve?
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