HVAC selector switch

95 Blazer

Air comes out the defroster rather than where the selector switch is set. I can hear the air hissing. I took the rubbery diaphragm off the switch, put it back on, and no change. I go back out two hours later, and the switch works again.

Can this switch be cleaned / resealed / fixed or do I need to buy a new one?

Thanks in advance.

Manual HVAC or Automatic Climate Control?

Air flow defaults to defrost when the system looses vacuum. Check vacuum reservoir, vacuum check valve, and the lines connecting them (under the hood).

It has the manual HVAC control.

I have vacuum at the control switch.

Tee a vacuum gauge into the HVAC line under the hood, before it goes to the interior. Start the engine, allow vacuum to build, shut the engine off... does the vacuum hold steady?