I have a 1991 Cutlass Supreme.

The defroster control does not change the air flow from defrost to heater or heater to defrost. I'm getting a constant air flow thru defrost and constant thru heater duct.

I have changed the heater control from the salvage yard. It still acts the same way.

I suspect a vacuum leak but it is very hard to locate the main vacuum line coming in under the instrument panel. I think I have found the vacuum controller under the right side of the dashboard. I pulled the set of four or five lines off it and checked for vacuum and couldn't find any feeding in there.

The manual talks about a vacuum solenoid valve and an A/C Vacuum Solenoid Module, but doesn't give many details on if all three are the same thing or at different locations.

In addition, I spotted a vacuum line going into a steel pipe that I think goes back to the rear of the car into a canister in the back left corner by the bumper. There are two lines attached to this canister. Is this part of the ABS or possibly something else? The lines going to it have rusted and broken.

Vacuum at the manifold measures about 15 PSI as I recall from a test I did.

Three questions:

1. Can someone tell me where the vacuum line comes in under the dash?

2. What is the canister under the back of the car?

3. The car stops fine. Should I just block off the line that looks like it is going to the back?

Any other ideas are welcome.

Thanks!