Replacing The Toilet Water Fill Valve.
Two common problems with toilets are;
This pic shows the water level a little above the water mark
If the water level is too high, it may allow the water to overflow into the overflow tube. This would be one reason a toilet would run continuously.
The pic above shows a spring clip adjustment instead of an adjusting screw.
If your toilet tank water fill valve does not have an adjusting screw, or if it is corroded or otherwise not able to be adjusted, you may need to replace the whole valve.
Replacing the toilet tank water fill valve is something any DIY er can do.
First, shut off the water to the toilet.
Before you remove the water line going to the tank, be sure to have a small bucket and a rag to catch any water that may run out of the tank or the water line.
Next, flush the toilet and hold down the flush handle down to allow as much water as possible to escape the tank into the bowl.
Now you have two choices, either remove all the water from inside the tank, or be ready to catch the water as it flows out of the tank when you remove the fill valve. (If you do this, make sure your bucket is large enough.
Now you're ready to remove the water line going to the water fill valve.
Be prepared for the very real possibility that the shut off valve may not work, or will only partially shut off the water as needed. (You may find that you have to shut off additional valves thru out the system to get the water to shut off to the toilet.) This then would be a good time to also replace the shut off valve if needed.
With the water shut off to the tank, now remove the water line going to the fill valve and then remove the nut under the tank that holds the fill valve in place. Remove the fill valve. (Slowly, if you've elected to just let the water drain out of the tank at this time.)
The new fill valve will have a new rubber washer with it that may look a little confusing. They come with a center piece that needs to be punched out and thrown away, leaving a round rubber washer that will obviously fit around the body of the valve.
The washer ready to install onto the valve.
This rubber washer will be installed onto the fill valve body before you place it into the hole in the tank. (The washer seals the water from leaking out from inside the tank.)
Position the valve in the tank so as to allow the little rubber tube (supplied) to be able to extend and drop into the overflow tube inside the tank.
Now, reattach the water supply line to the bottom of the fill valve and slowly turn on the water. Watch to see that the water fills to the proper level using the adjusting screw to achieve desired results.
(A word of caution -- if your water supply line is one of those hard chrome metal lines, replace it with a metal wrapped flexible line made for toilets. If you try to reuse the old metal line, you'll have a hard time getting it to seal tightly.) Use the metel wrapped instead of plain to help prevent future blowout.
That's it, you just saved at least $80.00 over calling a plumber!