Hey Tom it seams you have some good answers to the same problem I am having but my house is new and all 7 bathrooms are full baths. 2 bathrooms are low on water level when they flush, the bowl stays with 3 inches of water. On the same floor I have 2 more bathrooms, those are keeping a good level of water... perhaps 8", I have two vent stacks on the second floor, three bathrooms are sharing one 3" vent stack, the other 1 shares another 3" with a 1st floor bath and a laundry... Why am I having the problem on two of the three bathrooms.. BTW all water levels were preformed before I set them and held water perfectly, not one drained an inch...

Tony