With my Roadmaster at random, the HVAC will blow to the main upper vents, and I cannot select middle, blend, or floor. I have done no diagnostic testing, other that I replaced the HVAC control unit (I had a spare). I am sure this is not a vacuum issue, but an electrical issue! It could be a wire is jacked up, or I need a new "vacuum electric solenoid". I plan on doing diagnostics, ie checking the circuit. I do get the random code 26, but I don't think that is related; maybe another matter. I couldn't find any postings about this.