You got 2 choices now:
1 - The easy way - Disable the vacuum. The only real purpose of the vacuum line is to make the A/C work a little better, and that's only in MAX AC setting. This is really easy to do, just go to the vacuum tree on the manifold and find which one goes into the dashboard. Remove and plug the vacuum port.
2. - The hard way - The vacuum is controlled by the heater control panel. I would guess there is a displaced seal or it's just plain broken. Check it out, fix or replace. Junkyard part or you can get it new from LMC truck for $130.
I want to clarify something for a future reader:
The door that bludmon is talking about controls whether air is recirculated through the cabin or fresh air is used from outside. The main reason for this goes back to the idea of MAX AC. If you can imagine a car parked in the summer sun all day, it gets really hot inside, maybe 110 degrees. Now, you could try to cool that air by recirculating it through the AC, but it is more effective to use outside air, which is cooler than 110 degrees to start with.
The same vacuum line goes to a manifold valve on the heater hoses. This is only on 93-94, by the way. The idea is on MAX AC or OFF, the system shuts off flow of coolant to the heater core. There is apparently a shortcoming in the design, and heat from the heater core reduces the effectiveness of the AC. So, for MAX AC, they added this gizmo to shut off the flow of coolant.
This may be an un-needed complexity for some. I'm in cold climate, so I never use MAX AC, but I want to maximize my heat. So, now that I understand how this thing works, I'm taking it out and just run straight heater hose from the engine to the core. Probably need new hoses anyway.