Toilet keeps running slightly when it should shut off . Do I have to replace the entire float mechanism?

Check to see if the float is hitting something or rubbing on the side of the tank as it comes back up. If so, if the float arm is metal, bend it slightly to clear obstructions. If plastic take old of the column it is attached and rotate it slightly to clear. Becareful, old plastic parts can become brittle, so handle with care. If you have the float that slides up the column, check to make sure the surfaces are smooth. Finally, the filler mechanism can be replaced after shutting the water off to the toilet and draining all of the water. Replacement instructions come with the filler. It is easy.

hi Patricia.
Overview Toilets haven't changed much in the last 80 years. After a flush, water still fills a tank, lifting a float that shuts off the water when it reaches a certain level. A lever still opens a flapper to cause the flush, falling back into place when the water level drops. So it's no surprise (nor any consolation) that we face the same flush problems today that our grandparents did. Sometimes the flush is too wimpy, sometimes the toilet keeps running, and sometimes the bowl doesn't refill.
Our ace plumbing consultant has a simple four-step strategy to solve 95 percent of these problems. You can complete the first three steps in five minutes. That'll solve most problems. The fourth step is usually easy too, but not always. More on this later. These steps work for most toilets but not for pressure-assist models.
Check the fill tube

Overflow tube problem

Push the fill tube firmly onto the fill valve. Make sure the fill tube sends water into the overflow tube.Remove the tank lid and find the fill tube. It's a small flexible tube that runs from the fill valve to the overflow tube. While the tank refills, this tube squirts enough water down the overflow tube to refill the bowl after the completed flush. If this tube falls off or the water stream misses the overflow tube, the bowl won't fill and your next flush will be wimpy (that is, won't develop a strong siphon). Reattach the fill tube and make sure it perches about 1 in. above the rim of the overflow tube. Flush the toilet and watch the water stream to make sure it goes down the overflow tube.
Adjust the fill height

Check the float

Adjust the float to set the water level. Pinch the clip and slide the float up or down on the rod. Keep adjusting the float until the water shuts off at the proper level.The water level in the tank is controlled by an adjustable float. A float that's set too low produces a weak flush; if it's set too high, water spills into the overflow tube and the fill valve won't shut off. The toilet keeps running. Look for the fill level mark on the inside back of the tank and mark it on the overflow tube so you can see it more easily. If you can't find it, measure down about 1 in. on the overflow tube and make a mark. Then flush the toilet and see if the water reaches and stops at that mark. If not and the toilet keeps running, adjust the float up or down. If you have an old toilet, you'll have to bend the brass rod that connects to the float ball to make adjustments. But with newer toilets you usually turn a screw or slide a clip along a rod. Flush the toilet after each adjustment.
Also make sure that the water level is at least an inch below the C-L (critical level) marked on the fill valve. You can adjust the height of many valves to raise or lower the C-L.
Occasionally the fill valve simply won't shut off, which means that it's defective. If so, turn the water supply off at the shutoff under the tank. Buy a replacement valve (sold at hardware stores and home centers). You don't have to match the old one; many, like the one shown, fit most toilets. It's a 15-minute change-out.
Adjust the flush handle/flapper chain

Flapper chain fix

Adjust the chain to leave a little slack with the flapper closed. Then cut off the excess, leaving about an inch.A chain that's too short or tangled won't allow the flapper to close and water will continue to leak into the bowl. This causes the fill valve to cycle on and off to refill the tank. A chain that's too long, or a flush rod that hits the the tank lid, won't open the flapper wide enough to stay open for the full flush. You'll find yourself having to hold the lever to complete a good flush.
To avoid these problems, adjust the linkage in the chain to leave only a slight bit of slack when closed. Cut the chain at the rod to leave only about an inch extra to reduce the potential for tangles. Then put the tank lid back on and make sure the flush rod doesn't strike the lid when you press the lever. If it does, bend it down slightly and readjust the chain.
Replace the flapper

Change the flapper

Unsnap the old flapper and take it with you to the store to find an exact replacement. In addition to the closest replacement, pick up a "universal" type.If you've completed the first three steps and your toilet keeps running, chances are you have a worn-out flapper. Turn off the water, remove the old flapper and take it to the store to find an exact replacement. (Hardware stores often carry a wide variety.) Most flappers snap over ears on the overflow tube. Others have a ring that slips over the tube.
Now here's the catch. You may not find an exact match. The range of flapper styles has mushroomed over the last 15 years, and you may find 15 to 20 flapper options on the store shelf. Some packages include specific brand and model information (so note yours before you leave home). Others have a "universal" label. If you can't find an exact replacement, try the closest one and pick up a universal type as well. They're cheap, and the extra one just might save you a second trip to the store! (Avoid the "adjustable" types unless you're replacing an adjustable one.)
Install the new flapper and make sure it opens and closes freely. Then test it. If the toilet keeps running or runs intermittently, you're not getting a good seal. Try a different flapper if the toilet won't stop running.
If you just can't find a flapper that seals, consider replacing the entire overflow tube/flapper. On most toilets (two-piece), this means removing the tank. It's not difficult and you don't need special tools. It'll take you about an hour, and you'll avoid that expensive plumber service call.
if you go to this link it will show you step by step on how to fix it. How to Stop Running Toilet Family Handyman

Sounds like your float may need to be repositioned. Try bending the rod that supports the float so the float is further down in the tank. If this doesn't work then you need to buy a replacement kit for the shut off assembly. It is all one piece and includes the float and is easy to replace.

The mechanism controlling the water is worn. Replace it if you can't get a kit to replace the worn parts.

It could be the type of flap you are using. I would suggest you try a different shaped flapper (a cone would work best) and if that doesn't do it, put a new kit into the unit, because it could be that the float system is sticking causing it to continue running.

If there is a continual leak in to the toilet bowl you will likely need to replace the flush valve or the toilet flapper.

As a first step try try cleaning the sealing surface of the flush valve to se if the flapper will seat and make a seal.

If this does not work and the flapper is visibly detioriated and breaking down shut off the water supply, remove the toilet flapper and visit your local plumbing supply store to purchase a suitable replacement. Install the new flapper, turn on water supply.

These illustrated instruictions may also assist:

http://www.danco.com/HowTo/How-To-Replace-a-Toilet-Flapper

Although less likely, if the sealing surface of the flush valve is damaged or detioriated proceed with replacement of the flush valve. Again turn off water supply and follow the instructions set out here:

http://www.danco.com/HowTo/How-To-Replace-a-Flush-Valve

If there is not a leak in to the toilet bowl and the water flow does not shut off completely, then water is flowing through the fill valve and the overflow pipe and you may need to adjust the float and float rod to allow clsoure of the fill valve. See here:

http://www.danco.com/HowTo/How-To-Replace-a-Float-and-Float-Rod

It's possible there's too much tension on the chain which won't allow the flapper to completely seal and let the tank fill. If this is not the case, the float may be stuck. If the float is sticking you're better off replacting it. If the tank is filling and water is overflowing into the tube, then it's the float.

Readjust the float. the longer it is the more leverage the fill water will have on it. good luck. The fill valve does come apart and cleaned if there is debri in it. Turn the water off first:)

You need to either bend the arm that connects tot he float down (if it is the type with the ball float) or adjust the height of the fill valve assembly. THis accomplished by slightly twisting the valve assembly carefully and sliding down a half of an inch or so and then twisting back to lock it in place.

This should fix you up right.

if its a metal arm connecting the ballcock shut off valve to the float flush it and bend the metal so the float is lower. if u can adjust the ball **** lower then do so. the shut off is not shutting off cause the float isnt going high enough. and the excess is going down the overflow.