VS climate control vac leak

[immortality]

i have a vacuum leak somewhere in the climate control system. if i run the climate and shut the engine down but turn the ignition back on the system reverts to blowing at your feet after about 20 seconds. this also happens sometimes when driving and using high throttle setting for long periods of time.

if i run the engine and then shut it down without having the climate on then vacuum is maintained in the vacuum cansiter. this suggests that there aren't any leaks between the engine - vacuum cansiter - vacuum servo unit

i have tried running and then turning off the engine at all the different mode settings to see which port is loosing vacuum but they all seem to do it. i have pulled the center dash section apart and pulled out the vacuum servo unit but can't see anything obvious like a loose/split pipe etc.

i now suspect the actual servo unit itself. does anybody know anything about these? is this a common problem?

cheers in advance for any help/advice

[The1]

it's a common problem for the rubber seals to deteriorate internally in the vacuum solenoid it's about $130 to replace with a new one.