It is pretty easy to replace a toilet valve.
Simply shut the water valve off to the toilet, flush the toilet tank of water and place a bucket under the toilet tank to catch any water that may drip from the toilet tank.
Next undo the water supply line using some wrenches and then undo the nut holding the toilet valve to the toilet tank and hold the toilet valve with your hand to keep it from spinning as you remove the nut.
Next pull the old toilet valve out and place the new one in it's place and then put the rubber seal washer on and the nut and secure it but don't over tighten it.
Then install the supply line and tighten it and turn the water on and check for leaks and adjust the toilet fill valve float as needed.
The signs that you need a new toilet valve are constant running water in which the toilet valve never shuts off and keeps running, slow tank refilling, weak or incomplete flushing, water level issues, water leakage.
If you adjust the toilet fill valve and it still is not shutting off or it's not allowing enough water to flow into the toilet tank then it should be replaced.
If the toilet flapper is not sealing and is leaking water into the toilet bowl then it should also be replaced if it's old and not simply just out of adjustment.
A toilet flapper not sealing can be a result of a bad toilet flapper that is dried out and hard or the toilet flapper may be the wrong size or the chain is too short and not allowing the toilet flapper to go down far enough to seal.
Toilet flappers tend to last 5 years and need replaced after that time.
To install a toilet flapper start by turning the water off to the toilet, flush the toilet to empty the tank of water, then unhook the chain from the toilet flush handle.
Next grab one side of the toilet flapper where it hooks onto the toilet flush valve assembly and snap it off or in some cases they simply pull up and off.
Then do the reverse to install the new toilet flapper by snapping the hinged part over the toilet flapper or flush assembly one by one until they are snapped on firmly and lower the toilet flapper down into the hole to seal it.
Then adjust the chain link to ensure it lifts the toilet flapper up high enough and also seals when it lowers.
To fix a faulty flush valve you can sometimes place some petroleum jelly on the underside of the toilet flapper which will sometimes seal the toilet flapper.
But it's always better to simply replace the toilet flapper with a new one as they are cheap and easy to install.
In some cases it may be the chain is too short and not allowing the toilet flapper to close properly and in that case adjusting the toilet flapper chain can fix the faulty flush valve.
To test a toilet flush valve simply place some food coloring in toilet tank.
2 to 5 drops of food coloring will work and then wait for 10 minutes to 30 minutes and look in the toilet bowl for any signs of the food coloring.
If you find any of the food coloring in the toilet bowl then the flush valve or flapper is leaking.
If it's a toilet flapper flush valve then you simply need to replace the toilet flapper to fix the issue.
If it's a different type of flush valve then the entire flush valve will need replaced but the majority of flush valves use flappers which are easily replaced.
Flush valves can go bad although they tend to last 5 to 10 years and sometimes longer.
If the flush valve is the flapper type the most common problem is the flapper itself which is cheap and easy to replace and they dry out overtime and don't seal as good.
If you have a flush valve that uses pressure those too can go bad and need replaced entirely and usually 10 years or longer.
Toilet fill valves also go bad eventually but are usually good for 10 to 20 years.
You cannot replace a flush valve without removing the tank although you can replace a toilet fill valve without removing the tank.
You do have to remove the toilet tank to replace the toilet flush valve.
The tank of the toilet will need to be unbolted from the toilet bowl to access the large nut on the toilet flush valve to remove it.
You should also use new toilet tank bolts with new seals otherwise it will leak.
Without removing the toilet tank you won't be able to remove the old toilet flush valve.
The difference between a flush tank and a flush valve is flush valves mix air with water to achieve more pressure while flushing to provide a cleaner water closet bowl.
Flush tank toilets feature a tank that holds water until a flush is initiated, at which point the water is released to clear the bowl.
Flush tank toilets have a fill valve that is connected to the water inlet that uses a float system to allow water to turn on and fill the tank and then the float shuts the water off when the tank is full.
A flapper in the toilet lifts up when you press the flush handle and allows water out of the toilet tank into the bowl to flush the toilet.
Flush valves usually use water pressure instead of a normal flapper to force water into the toilet bowl for a better flush.
Some toilets also use pressurized water tanks that force water out into the bowl instead of filling a tank with water which also saves on water.