A friend of mine lives in the last floor of a building with 4 floors. One of his bathroom syphons keeps being emptied each time he or a neighbour flushes the toilet. The water is simply drained out. I think the problem may be related with the small size of the air inlet at the attic of the building (around 2.5cm). Needless to say that an horrible smell comes with the empty syphon.
The strange fact is that this only happens in one of the bathrooms. To be sure I closed all the sewage pipes in this bathroom. The water in the other bathroom syphon is stable when flushing the toilet.
Best Answer
Install an air-admittance valve on the sewage side of the trap.