If you have a single handle shower valve, and water runs out of your shower head when the knob is turned off, you should replace the cartridge. What if you replace the cartridge and it still leaks or drips?
On a single handle shower valve, there really is only one part to replace…and that is the cartridge. There are a couple possibilities that may explain the leak.
The cartridge is a cylindrical part with a variety of o-rings and rubber pieces on it. The fit is snug. It may be that as the cartridge was pushed into the opening an o-ring may have twisted and allows water to pass. It may also be that the cartridge is bad. In any case, you will need to remove the cartridge.
Once you have the cartridge out, inspect it for defects. It really doesn’t take much for water to pass. Look for tiny defects or nicks in the rubber parts. Also look for cracks in the plastic housing. If everything looks fine, rub some liquid soap on the cartridge and slide it back in. The soap acts as a lubricant and so the part should go in very easily. If it still leaks after this, replace the cartridge with a new one…there are times that you won’t be able to see the problem even if the part is new.Handyman Hints to Organize Your Garage » « Door Casing Reveal