Ford Repair: Ford AC Problem, motorhome, hoses

Question
Hi Tony,
I have a 20 year old motorhome built on the Ford E350rv chassis.  I have a problem with the cab AC (Not the motorhome roof AC).  It started with the AC blowing hot air when we knew it had been properly charged...the mechanic installed a cutoff valve above engine to shut off the hot water in the summer and allow the AC to work properly...it wasn't our first choice, but it works and he thought it would be less expensive than fixing the valve or blend door that was causing the problem.  

Now I notice that when I'm accelerating, the air coming out the vents stops and is forced somewhere else for a moment..when I let off the accelerator, it returns to the vents.  

I'm sure it has something to do with a vacuum issue, but if I could isolate the problem and buy the correct part, I would do that and have my mechanic install it....saving us lots of time.

Thank you!

Answer
Hello David, It does sound like a vacuum issue from description and to repair it you're going to need to check hoses from engine to interior,hoses inside dash for the motors on the plenum case and to the control head. that is where I would perform 1st and go from there. Thanks Tony