Dodge Repair: no air flowing from a/c dash vents on accelerating, dodge ram 2500, vacuum problem

Question
I have a 99 Dodge Ram 2500 V10 and with the A/C running, when I accelerate the dash vents shut off. When I take my foot off of the gas pedal the air flows. Sounds like a vacuum problem but where do I look to find the problem?
Thank you,

Answer
This is a common problem with vacuum operated HVAC air door actuators but unfortunately there is no common part that fails. Obviously there is a loss of vacuum a good place to start would be to replace the vacuum check valves for the HVAC system both at the engines intake manifold vacuum port as well as the one on the vacuum hose which passes though the firewall near the A/C lines.