Air Conditioning blend doors

[ttp17]

I have just finished restoring my 1968 Cougar, 302 cid/4bbl., with factory restored A/C. The blend doors for the the A/C move when I accelerate or decelerate. For example: With A/C on MAX, air blows through the dash vents normally. But if I accelerate like passing on the highway, the blend door(s) cycle and air comes through the heater vents. New vacuum canister. Good vacuum check-valve. New vacuum lines.

Any one have an idea on what's happening here?

ttp17

[xr7g428]

Bad vacuum check valve or vacuum leak. Minus vacuum, the motors return to the default position (heater or defrost).