I had a problem with my AC/heater control system on my 2002 Ford E-150 van. Turn out it was the vacuum check value. This valve is located under the heater box fan. It has a short vacuum line to a vacuum canister which is not accessible. The vacuum line goes thru the 
firewall near the right side lower kick panel. I removed the vacuum line in the inside of the van and push the firewall plug towards the outside with a screw driver. It cannot be put back in so you have to use some butyl tape to reseal the line so there is no air leakage into the vehicle. Tugging on the line released the valve from the vacuum canister. This valve allows the vacuum canister to hold vacuum on the system so the damper doors do not go to the rest position under acceleration, the doors will go to rest position if the valve or line is broken. Once the line and valve was off the canister I pulled up the line to get assist to the valve. I changed the valve and placed a long line of Napa vacuum tubing H452 5/32"X 6 feet tubing on the vacuum canister side with a golf tee in it. I pulled the new assembly back down to the firewall plug and reconnected the lines, adding butyl tape around the firewall plug. Butyl tape is what us auto glass installer used back in the ‘70s. I’ll try to post a photo of a clip of the front firewall that one of my body shop had, regards Rob
So the problem was a vacuum-motor controlled HVAC vent door not operating correctly b/c of a faulty vacuum check valve? My old VW Rabbit had a vacuum canister that looked like three tennis balls glued together . I don’t think I ever understood what purpose it served on that car.
The canister holds vacuum due to the check valve. When you accelerate the correct vacuum is lost so the check valve closes and then the system relies on the vacuum in the canister to hold the damp doors in the correct position of the dashboard switch. Unfortunately the check valve cannot be reconnected to the canister due to its placement.
Before I did the repair the doors would flop back to rest position when I accelerated in a few weeks the short hose that was the connection between the check valve and canister failed so the doors would not work at all. I believe the hose deteriorated and caused the check valve to fail first; later deteriorating to a point which let air in. You need the canister or long hose to hold vacuum while you are accelerating. You cannot see what is in the photo it is buried under the heater box. I knew there must be a canister down there but could not see it.
Others have redirected the vacuum hose to inside of the car by the dog house and put a canister in the inside of the vehicle however some splicing of the hoses needs to be done that way, regards Rob
Thanks for the explanation. Sort of like in 50’s and early 60’s cars where the vacuum powered windshield wipers would stop working when you accelerated.
Your right about the older cars. Todays newer cars will compensate for the vacuum leak in that little hose. Older cars this would mess up the carburetor vacuum and cause a idle problem.