Leak at vacuum breaker

[PAL07]

89 25ft Excella which recently began to leak slowly at shower vacuum breaker when city water connected and pressured up. This occurs when the shower manifold is turned off completely, which sounds wrong to me.
Is manifold bad, or, am I overlooking something simple?

[AtomicNo13]

I'd first try an overnight bath in something like CLR to break up any mineral deposits. There is a little flapper in there that gets gunked up. Easy fix.

[AtomicNo13]

It is designed to leak whenever you shut off the water.... Evacuates the line.

[PAL07]

Yes, but this leaking occurs with shower turned off completely, and not having been used. My understanding is the water evacuates the line only after being run through shower hose.
It appears that a trickle of water is being fed to the shower hose, even with the shower turned off.

[idroba]

Sounds like your shower valve is leaking and there is so little pressure on the vacuum breaker it then dribbles the water out. The cartridges are available, probably even free from the manufacturer, most have lifetime warrantees. Or just buy one locally, probably about $20 or less.