My toilet won't stop running, the float goes to the top but water continues to run. What's happening?

Check the tank water level, it should be at the mark in the tank or 3/8 to 1/2" below the top of the over flow tube. Now look at the flapper, If it has wavy edges on the flat surface, replace it. The linkage between the flapper and the flush lever should have 1/4" play when the tank is filled. Also, check the small tube coming out of the top of the ballcock and make sure it's connect to and draining into the white overflow tube. If the leak is at the base of the tank there are two brass bolts on both sides of the flapper, if the tank is loose, leaking or wobbly, snug up the bolts. If there is dampness at the base of the bowl, pop off the china caps and snug down the closet bolts. I would check all this out before paying 50$ for a service call and 50$ a hour thereafter. More questions? I'm as close as a click. TOM

One of the 2 things are happening:
1. Fill Valve has dirt under washer preventing positive seal
2. Your water lever inside tank is set too high. Water overflows into the overflow tube and inside the toilet never reaching height needed for fill valve to shut down.

Solution:
1. Close water supply for the subject toilet.
2. Reset water lever to correct height. ( about 1" below the top of overflow tube)
3. Now, depending what type of fill valve you have: you have to dismantle the upper part ( where float arm attaches to) and clean that washer. However, once you have it open, I would sugges you replace the washer).

Also, you can install new fill valve, like Fluidmanster A400. They are inexpensive , can be purchased in Home Depot and installation itself is a snap.

( Oooops, answering at the same moment again, Tom... :-D )