If you have recently replaced the tank flapper and your toilet is still running, it may be time to pull out the big guns and replace your ball cock, aka fill valve. I prefer the Fluid Master Brand fill valves over the old style floating ball types. They are much easier to adjust and tend to have fewer problems in the long run.
Step 1: Shut off the water supply to the toilet, hopefully using the shut-off valve located below the stool. Flush the toilet and then use a sponge to soak up the remaining inch or so of water in the toilet tank. Place some towels on the floor below the tank to soak up any residual water that may leak out during the repair process.
Step 2: Remove the old fill valve. Begin by unscrewing the water supply line beneath the tank. Use an adjustable wrench to loosen and remove the nut located between the supply line nut and the tank. Inside the tank, disconnect the fill tube from the over flow pipe and lift out your old fill valve.
Step 3: Using the water line marks inside your tank, adjust the new fill valve to the appropriate height. Make sure the fill valve does not touch the sides or top of the toilet tank. Reference the fill valve instructions for other necessary adjustments before completing the installation.
Step 4: Place the shank washer on the fill valve shaft, insert the fill valve through the base of the tank and hand tighten the lock nut beneath the tank. Tightening the nut too much could crack the porcelain, resulting in the need for a whole new toilet, so be careful.
Step 5: Once the new fill valve is positioned and secure, reconnect the water supply line below and re-attach the fill tube from the fill valve to the overflow pipe. Follow the fill valve instructions to make any additional adjustments, turn your water supply back on, replace the tank lid, pick up your tools and flush away at will.