I have two "odd" behaviors in a newly plumbed three-valve tub/shower (Price Pfister).
1. A fairly strong flow of water coming out of the tub while the shower is in use. (This made me initially suspect the diverter valve.)
2. When the water is turned off, it continues to drain out of the tub for almost a minute of a _slowly_ tapering to nothing flow. This happens with both hot and cold, and is accompanied by a lovely "gulp-gulp" sound. It does stop eventually, which makes me believe that the hot/cold valves are properly working.
Through reasons too long to explain here, I actually have another whole shower set (same model even), so I swapped out the diverter valve and found a suspect "bonnet washer" but the replaced unit had no such defect. However, the exact same behavior still occurs...