For a couple of years my Sharan had a slight hissing noise from the brake pedal. My trusted mechanic checked it out, found no obvious leak but reckoned it was harmless. A month ago it suddenly became much louder, like a leaking balloon, brake pedal stiffened and he replaced the servo. At the same time as the increased noise the car seemed to lose power, no acceleration beyond about 40mph. At first he thought this was somehow connected but the fault remained after brake repair.
Computer showed up a faulty Boost Pressure Sensor which he replaced to no effect. He has checked for leaks, tried replacing the MAF, loosened off the VNT vanes which seemed stiff but nothing works.
Does anyone have any idea where the fault might be?