That sounds like a venting problem. The water flushing from the toilet cannot draw air to equalise the pressure so is in effect trying to draw air from the bowl causing the water level to drop, if that makes sense. Fitting an air admittance valve should solve the problem. How easy it is to do that is on your pipework I don't know.
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by KitchenGuy » Wed Sep 18, 2019 10:34 pm 
