How to fix a toilet that runs constantly
running toilets and how to catch up with them
First you need to determine why it is running. You can lift the tank cover on most home toilets and look inside. At the bottom there will be a flapper valve. It is a simple rubber flap that covers the outlet hole. There will be a chain that runs from the flush arm to the flap. Look at the flap and make sure that nothing is obstructing it and keeping it from closing all the way. If it is working and the tank is full you want to look at the shutoff valve. It will have some sort of float that closes it when it reaches the proper fill level. Lift up on the float and see if it shuts off. If it does then bend the arm a little so that the float is lower and puts more pressure on the valve.
If these don't do the trick you will most likely need to replace the mechanism. It is a pretty simple job and a plumber will charge a lot to do it so I say do it yourself. Turn off the water and take out the defective part or parts. Be sure to take the parts with you to your local home store so that you are not relying on memory to match them. They will come with complete instructions to reassemble the mechanism. Just follow the instructions and guess what? You have just saved $50 - 100 that a plumber would have charged.
If your toilet is a tank less variety you will have to replace the Sloan valve. These are more complicated. Turn off the water then examine the piping above the toilet. There will be a pipe cap with a large hex nut. Remove this and remove the Sloan valve. Take it to the home store to match it. Replace the entire valve. These are generally only used in commercial applications and will have little in the way of instructions so this may be a job for a plumber. But again they will charge a lot so at least try to do it yourself you will find all of the parts at you local home store and while there they should be able to tell you which parts are bad. Make sure that the parts you buy come with instructions or get advice from their plumbing department before you leave.
The most common reason for failure of toilet, and most other, valves are worn out rubber seals or calcium deposits. So inspect all rubber parts for excessive wear and clean all valve parts. If you can see white deposits try soaking them in vinegar or spray them with some sort of "lime away" cleaner.