I need help figuring out what to do with my bathtub drain. It leaks when draining. Water seeps out from the bottom of the tub to the bathroom floor. My house is approximately 35 years old & built on a slab. I've found that if you pull the drain lever up about halfway & let it drain very slowly, it doesn't leak as much as when draining full force. Not an experienced DIY'er - where should I start?