Dash A/C Blowing HOT air after engine is hot

My gasser...the a/c cuts out sometimes when climbing a hill or under certain loads. But EVERYTHING cuts out. Fan, too. It's the vacuum. The vacuum cylinder is not large enough to to do the job under some stressful conditions. So the computer, I guess, shuts the system down to give the motor what it needs.
I was told that Ford had a larger vacuum cylinder for sale...and I figured, "OK, next year I'll put that in." But..."next year" comes and goes.