Good point...didn't think of that....but that would also lead me to believe that maybe that is the problem. Being so close to the drain, gunk has got into the check valve and is preventing it from opening.
I use something similar in my regular job...but we call them vacuum breaks. We also put them up fairly high to keep the hot water away from them (they can be subjected to steam on a regular basis)
I did consider that the vent was causing the issue and tested that theory in the beginning, and yes, water rushed out. So it wasn't the vent. As noted earlier it was a heavily clogged pipe (concrete slab foundation). $85 well spent assuming I don't have any recurring issues. The house is 17 years old and we just bought it last year. I'm thinking that clog I just had was the tipping point of a building clog over 17 years.