How to repair damaged enamel inside the toilet bowl

The water in the toilet seeks the level of the highest level of the high level of the trap, so if you raise the toilet bowl you must raise the trap the same distance. The trap is the curly, s-shaped part of the drain between the exit tube of the bowl and the wall. The highest level of the trap follows the line of bottom of the inner circumference of the trap where the the direction of the flushing water is parallel to the trap, as it is coming out of the climb from the bottom of the trap.

It could also be a vacuum in the drain sucking the fluid back into the drain if there is not a vent pipe running vertically from the drain to the roof to equalize the presscure.

I am not a plumber nor an engineer but a do it yourself person with a little math understanding from American public schools and a very basic, traditional grasp of physical science from experience and from the instruction of a peculiarly motivated chemistry professor at Bakersfield College, Ca USA.

Given a recent question, you might want to consider replacing the whole toilet. There are good examples of low-water-use toilets out now that reliably flush with far less water (1.28 gallons per flush) than older toilets, and you know all the parts will fit.

Otherwise you are looking at replacing, in order of ease, the flush valve or flapper (usually a rubbery part - they get old and crack or stiffen), or the flush valve seat and overflow tube (which often comes with a new valve) and pretty much always requires taking the tank off the bowl (cistern/pan for the across the pond folks.) Since you have to take the tank off the bowl, you'll want replacement gaskets for the tank to bowl screws.

That is assuming that you don't have a very simple issue such as a mis-adjusted float causing water to go over the top of the overflow tube - where you'd need to adjust the float, or replace a faulty fill valve if it can't be adjusted.

Aside from reduced water use, the main thing that buying a whole new toilet gets you is freedom from the multiplicity of different makes and models of toilet with non-interchangable parts. You want to know the brand (at least) of your toilet when you go shopping, and having some measurements or taking the defective part with you can help to sort out which parts will actually fit your particular toilet.