I am reaching way back but here...

The car probably had a vac leak in the climate controls system. The most common faults are:

Bad check valve.

Leak in the connection of the vac line from the engine bay to the "tennis balls" that are the vac reservoir.

Actual leaky reservoir.

Leaks at the connections at the selector.

A leaky actuator.

I think that is it. You have to trace the leak. Start by pulling out the black and clear oreo looking thing in the vac line before it passes through the firewall...blowing on it, air should only pass through one way. After that, apply vac to the line exiting the check valve and see if it leaks down...rinse and repeat through each piece until it holds vacuum.

I would bet on multiple cracked hoses (just the ends) at the selector or a failed check valve.

Mike