Because diesel engines don't develop an induction vacuum in the way a petrol car does, if you're having vacuum/servo probs, it may be the engine-driven vacuum pump is faulty. Follow that tube you've found round to the front left corner of the engine block (you'll need to remove the big plastic cover), where it goes into the pump that's sitting on the side of the block. Check the connections to that -- mine, the clip had come loose
Pete