'94 ventilation blend door - vacuum?

I get the A/C air blowing at my feet when the engine is under heavy load, so I know the blend door isn't in the right place, probably due to some vacuum issue since it's fine when cruising on a level load. THe question is - where do I look for the problem? Engine vacuum hoses? At the door itself? How would I access it?

There is a canister under the hood that stores vacuum for low-vacuum situations such as hard acceleration. It is fairly common for these to fail as cars get older due to either moisture intrusion or age. It is also possible that you have a cracked vacuum line leading from the throttle body to the canister, or the check valve/tee in that line is leaking. The canister is mounted to a bracket behind the drivers side headlamp.