If your toilet will only flush after a rapid series of pumps of the handle Our used to do that and now doesn’t flush at all) it is almost certain that the syphon diaphragm (sometimes called the flapper valve) is split.
The good news is that this is a simple thing to fix and not a reason to call a plumber out if you feel happy with DIY tasks.
What is a toilet flapper valve?
At the bottom of the cistern is a simple device with a flat sheet of thick plastic that acts as a valve, pulling water up into the siphon and then down into the flush pipe when the handle is pulled.
When this plastic flap gets split the amount of pull available lessens. For a while it still works after a number of pulls. However once the syphon diaphragm has split it will only get worse and will eventually fail completely.
How to fix the toilet syphon diaphragm
To fix the syphon diaphragm you need to remove the entire siphon unit. Firstly empty the cistern of water and turn off the supply to stop it refilling.
Bail out ALL the water so you don’t get soaked when you undo the large nut beneath the cistern.
Once you have done this you will find another nut which attaches the siphon unit directly to the bottom of the cistern. Undo this and the siphon unit will now be free. Look inside the cistern and detatch the handle from the siphon unit which you can now take out. Now replace the diaphram (costs less than 50p from a diy merchant) and reassemble (seriously, it is so easy there is no need for me to tell you specifically how to do this)
When reattaching the siphon unit to the cistern, apply ptve tape to the threads to ensure a watertight seal.
Tighten the big nut using a pair of plumbers grips such as these from Amazon.
That should solve the problem The only other major issue people find with toilet cisterns is when they don’t fill properly. In this case you need to replace the float valve. That is the other unit in the cistern.