We are replacing two upstairs toilets with Toto models (haven't finalized choice yet). Not doing any other renovation at the moment. Replacement to be done by plumber.

One toilet often loses almost all the water in the bowl; the other toilet never does. It seems to be related to dishwasher use (but not consistently -- could there be multiple factors?) I read on the FAQs that this could be caused by a blocked roof vent.

Would it be better to have the vent issue fixed first, before the new toilets are installed, or does it not matter what order they're done?

[also, may I say I highly appreciate the courtesy and enthusiasm shown on this board, a world of difference from many of the newsgroups, and a credit to you, Mr. Love -- thank you.]