I found these pictures on the internet where you can zoom in on the check valves. Yours seem to be orientated the same way. These valves will keep the overpressure in the intake manifold (from the turbo) away from the vacuum system and only let air pass if there is a vacuum in the manifold (eg idling or when engine braking).
You have indeed the ECC system, in which case the shutters are still vacuum operated, but operated by an electric valve unit, controlling the vacuum to the vacuum servo's (did you check fuse 16?).
See enclosed diagrams: as you have been fiddling with check valves item 10 and these are a common point for all valves, it would be my starting point for fault tracing.