Thanks for that Graham, The "supply" to the solenoid valves is direct form the manifold and the diagram shows the check switch as a vacuum switch so hopefully I am right (but you never know for sure !!)

When you apply vacuum to the hubs from the mityvac they seems to work as it pulls the gear onto the drive shaft to lock the front wheels to the shaft and a magnet holds it away from the shaft.

Forgot to mention I have checked all pipes with smoke machine for leaks etc