You can't alter the water level in a toilet bowl. That depends on the design of the ceramic bends and soil pipe connection in the toilet.
You might be able to alter the flush volume for small and big flushes. But the water left in the bowl will remain the same.
How to depends on identifying the make of the flush valve for the instruction leaflet. That one will undo on a bayonet fitting for removal and examination (close fill valve stop tap first).
As above, gravity determines the amount of water left in the pan after you flush
Water, irrespective of the volume, will enter the pan, and leave because of curves/gravity.
I too suspect that you are asking how to increase the volume of water that clears the pan. I am not a plumber. I don't recognise the mechanism that you have though, and will defer to the pros. Best of luck
We moved last year. The toilet we had then was a modern one and honestly, it would often take two flushes.. However, the new toilet we have is a 'classic' high flow toilet. Honestly, one flush and the big jobbies is gone! Jurassic park!
If the level in the bowl has decreased over time, it might be caused by a partial blockage in the downstream pipework, which can give a siphon effect. Check the manholes.
In the UK, majority of pans are P traps. The level in the pan is determined by the P trap outflow level. Once flushed the water in the pan will always level out to the same as the lowest point of the toilets outflow spigot
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local