Thanks for the info, Charles.

I tested the vacuum on the brown line circuit and found that there was a small leak when the key was in the "ON" position. I replaced the vacuum switch with a new one and it now holds a vacuum at "ON" and "OFF", and bleeds off the vacuum in the "in-between" key position. I noticed that the new switch had a blue mark on one of the tubing connections. I connected this to the brown/blue stripe line that goes to the IP. I noticed that the old switch was connected with the line colours interchanged. I don't know if this makes a difference, but everything is working OK for now.

I get a vacuum of 11 inches Hg throughout the system now which is probably a drop in atmospheric pressure of about 5 psi.

Gary