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.