Re: Vacuum problem on a.c.
Assuming it is a falcon......I had the same problem with my auto CC in the XR8.
For me it was a leak in the green vac hose from the Vac dist module (the heater vac line). The high vac in the system at idle was enough to negate the leak, however when vac dropped off with the engine under load there was not enough to maintain the system, so the vents reverted back to default (windscreen).
I fixed this by blocking off the green hose outlet at the vac module. It means that the heater tap is always open, but it does not seem to effect performance of the climate control. Just means the system is entirely relying upon the blend door to control the temperature. Everything else still works fine.
I was lucky that it was the green line, as that is probably the one that is least important to the system.
EB II XR8 5sp
K & N filter
Short throw shifter
Just added climate control
Trip computer and