Passenger's foot well has the two little plugs that lead to the engine bay, I noticed that one is a little thicker than the other and originally one has a slightly fatter tube/hose attached to it than the other , not by much but enough to make me stop and wonder if there was a reason for it. Do they both carry air and no diff or does the fatter tube carry 'other' stuff .. hidden secret?

footwell wall-nipples

 

No secret - see below vacuum diagram. See BULKHEAD MOUNT below:

 

Right, thanks. SO the thickness of the tube is irrelevant in this case (perhaps replaced at some point) and they both circulate only air.?

 

The size of each tube is relevant to it's function: smaller tube comes from inside the cabin and goes to water valve and larger tube goes into the cabin and HVAC (et al) from the engine. You can't swap them around and get desired results. However, you are correct, in that they both are air (vacuum) lines.

Dave

 

Thanks. From memory the fatter of the two tubes was the water value > servo drive tube stationed at right looking towards the cab and at left looking in the footwell end.

 

I'm not sure my 1985 S3 is plumbed the same as yours. Here's a diagram for an S3.

Dave

 

Assuming that both nipples are created equal that they are just ports from the cab to the engine bay and neither port employs a hidden 1-way valve or such likes.
I guess only testing with the specific model would tell.