My delta bathroom faucet (the cold) won't turn off. Started as a small stream, now it is at full tilt. We have taken it apart and replaced the washer/seating and the cartridge.

It is quite old (about 17 years)... there is a hot/cold and a hand held shower as part of the set.

I assume your Delta faucet has ball-type cartridge. If so, than the ball may be misaligned inside the valve - or the guide pin has broken off. Remove the ball and reinsert. There is a guide on one side of the ball and pin inside the valve body. Make sure they align. Reinstall other parts.