Your 1985 PJK should have a triple-protection valve (as shown on the HMVF illustration) rather than the simple non-return valves on the main reservoirs.
Looking at the
http://www.truckandbusforum.com/gall...3/imgp9440.jpg illustration will help you (and your mechanic) get an idea of the layout.
Ignore the extra components for the trailer brakes and the winch!
Your air doors
should be fed from the third port on the triple-protection valve (which should be on a chassis sidemember somewhere between the condensing tank and the two main reservoirs).
It might be difficult to trace things unless you can get the bus over a pit or on a hoist, though you might have floor panels that can be lifted.