I have a single handle faucet for my tub shower. Recently for no apparent reason when the faucet is pushed in to turn it off, the plunger pops back out very slightly causing water to continuously run out. (Albeit slowly but more than just some drips) The plumbing is original. About 25 years old. The internal mechanics are made of brass. There is one large screw above the plunger wich is I assume some type of clean out. And two screws that branch off each side. I assume they are adjustments for the hot and cold. Is the valve bad or can it be adjusted/repaired.

Can you show us a picture?