Thanks, I found a photo where you can see the hose just clipped up so at least I know that it is a rubber hose. Yes I have "air brake vacuum hose". I used 3/8" ID because it was easily available and I adapted the end of the steel pipe with a 3/8" tail. The manifold port is 3/8" anyway and I've proved it more than adequate size.
The original pipe underneath is 14mm OD and about 10mm ID