hi mate, they dont actually have a dump valve they have a diverter valve. Instead of dumping the air out of the engine it recirculates the air back into the engine.
These are very straight foward to replace, they vary from 80-150 quid depending on what you want.
I think that the Diaphragm types are better as they are quicker, and respond faster, but they do cost more.
Hope this helps
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