No idea where you can get the diaphram. If you do a search for "ARV" in diesel discussion, there are several threads on how to disable it.

Your idea of how it works is a bit off. It's only for emissions, it has 0 performance benefit. More specifically the EGR function. They want more exhaust gas flow into the intake but the intake pressure is too high to flow engough. The ARV lowers intake pressure to a point much lower than the exhaust manifold pressure. This allows greater flow of exhaust gasses into the intake than before (non-ARV). Higher flow results in less NOx. Current EGR diesels use VNT turbos (increase exhaust backpressure) and intake flaps (lower intake pressure) to achieve this. A notable exception to this is the Cummins ISX, they use a VNT turbo and a very restrictive venturi tube after the intercooler.