Low vacuum to vents on acceleration

Everytime I give the 95 PA a bit of gas to go up a hill, the ac switches to blow out the windshield vents instead of the dash vents. I know this is caused by vacuum dropping causing the vents to go to their default position (windshiled).

I put a cauum pump and guage on the vacuum diaphram right next to the vacuum canister in the engine compartment and I can pull a vacuum on the system but it leaks down after a while. is this normal?

Should the vacuum to the heater/ac box leak down?

Im trying to figure where the problem is. engine going south (ie low vacuum) or leaky vacuum component somewhere.

I just put a rebuilt engine in the car last year so I'd be pissed to find the engine is getting weak. I have done a vacuum or compression check on the engine in a while but last time I did, compression was real good.

any ideas?

has anyone troubleshot the vacuum controls on these cars?