Just to add experience with another set-up, I'm using a Roadmaster Evenbrake in our 2005 CRV, behind a Monaco Monarch. We've towed 14,000 miles with it. I did have a lock up, twice, but it was my own doing. Fortunately we were only going 15-20 mph when the toad became an anchor. When the Evenbrake is set up, you go through a test in the car to be sure it is good to go. Then once moving, you do a quick short stop to cycle the system. Well I forgot to do the moving brake check, went over a little bump in the road and the Evenbrake locked 'em up. Scared the dickens out of me the first time. (Felt a little dumb the second time!) Other than that, it has worked as advertised and kept our stopping distances reasonable. I never thought about the residual vacuum in the CRV, I suppose the two applications that happen while setting up will bleed it off. As far a seat position, I agree there should be a little bit of room between the brake unit and the seat. If I get it too snug, the brake is overly sensitive.
Keith