Hi,
I have the exact need. I recently had a tub/shower converted to a shower only (new tile, floor, etc..) and the plumber re-used the existing in-wall plumbing because it was almost brand new. He moved the control valve higher up the wall and put a diverter for a hand shower where the tub basin spout used to be. I have since learned that instead of plugging the control valve for the tub spout, he simply used the same pipe and installed a diverter. I guess he didn't realize that the overhead shower would still function when the diverter was turned to the hand shower due to the back pressure when using the hand wand. When you unscrew the hand wand hose, it works fine: Water comes out the wand pipe and not the overhead shower; However, when you hook up the hose, the water comes out of both even though the diverter is turned to the wand. So when the water flows through the diverter unrestricted by a smaller hose or hand wand, it doesn't flow from he overhead shower arm. He and I think it must be because of just enough back pressure through the diverter valve to cause the pressure equalizer in the control valve to force water up to the overhead shower.
Short of making the plumber rip-out the wall tile and replumb this thing (neither one of us wants this at the point), does anyone have any suggestions. I saw the post about the Grohe vacumn breaker and I was wondering if I could simply put one of these on the hand held shower? I thought about experimenting with different hand wands to get one that wasn't restrictive but this is wastefull and could get expensive and still not solve the problem.
The overhead shower part works fine by the way.