Good day from Northern California

My wife and I have been long time DIY folk, 20+ years now. We are currently renovating our guest bahtroom down on the lower level of the house. It had rather ancient Speakman shower valve and my wife wanted to modernize the look a bit so we picked up a Moen shower set. I finished sweating in the shower and we turned up the water to the house. Yay for no leaks, except the valve doesn't seem to turn all the way off. It drips badly in the off position. Ideas on why a new valve would be doing this?