Could be simpler. If you remove the check valves in the engine bay (they break and need replacing over the years) you could have simply pressurized the lines that go under the passenger side firewall area under dash. There is a block there that run those vacuum lines to the HVAC controls. Mine did that. Easy fix, just plug them back in. Might be able to reach them through the glove box, not sure, it's been forever but thats the easiest problem to have. You do not want to be messing with the actual HVAC parts if you can help it.