When I push on the gas pedal, the a/c starts blowing out the defrost on what's left of the top of my dash! When it levels off, or when I let off the gas, it returns to the right vents. I figured it's a vacuum LEAK somewhere but can't seem to find it. Anyone had this problem? If so a solution would be much appreciated. Thanks

 

You are working on what year make and model?

Yep, vacuum leak. Might be defective check valve.

This on a Farmall tractor, right?

 

Check and or change all the hoses and vacuum canister/s....

 

I have the same problem with my 1998 Van... Just learned to live with it.. You will end up tearing the dash apart if you want to fix it to repair the blend doors.. There is a small inline check valve under the dash behind the control panel in one of the vacuum lines very slim chance it could be bad.. ..

 

I'd replace all the check valves and hoses. That should cure your issue.