Mark Williams
This is usually caused by improper installation. In the past, when a 2-handled mixer was used to supply a shower as well as the tub, the top outlet was plugged, then the tub (bottom outlet) was piped to a twin diverter ell. This device was not usually included with the faucet. the riser to the shower head was piped fron the diverter ell, and the tub spout is connected to the other side of it. More modern mixers have this diverter built into the faucet body, and dont require a diverter ell, BUT NOT ALL! If the valve in question was originally configured for tub or shower only, the needed internal diverter was not installed, and direct connection to the mixer with the shower riser and tub spout will always result in the dribbling you describe
