think its something to do with the vacuum line circuit that control the brakes and the turbo and the egr
never shown in its entirety in the parts diagrams .. two separate diagrams see below links
so you have to sort of guess whats going on ..anyway it all consists of one way valves, sensors etc to keep the turbo bit separate from the brakes and egr .
i would have to pull my own system apart to get even close to explaining it (not doing that)...you'll probably not find anything about it in the haynes manual.
if its inoperative in anyway ..its a matter of replacing pipes working out the valves
See that little pipe on the the one way valve here, pipe marked 2420, the valve is half way along it.
https://ford.7zap.com/en/transport/121/ ... 939/97085/I THINK - this is whats hooked up to it
https://ford.7zap.com/en/transport/121/ ... 757/81941/all the best.mark