The only time I get negative pressure (vacuum) is cooling down the carboy, that will suck the fluid from airlock or blowoff bucket into it. What I do now is not to fill airlock or bucket until the temp is at my desired ferment setting, then I add the fluid. After this the Co2 is exhausting the o2.
Bottled: Gewurztraminer, Pinot Noir, Cabernet Sauvignon, 16B, 18B, 18E, 19B, 20, 21A, 23, 27
BEER, a mixture of malted grains, water, hops and yeast, that is transformed through the course of fermentation that transcends the simplicity of those basic ingredients, fundamentally into the drink of life.