The Helium tank would be overkill- - - -those things are designed to hold 2K PSI or so and they're very heavy.
I don't mean the tanks similar to tanks used for welding... Maybe you haven't seen them at Walmart, but they remind me of a smaller 20# LP tank, but lighter weight construction. Either way, I'm really liking the PVC idea! What do you think would work size wise? 3", or 4" diameter by 12, maybe 18" long? What about some sort of pressure relief? As in, if I don't use my brakes/wipers in 20 minutes of engine runtime, will I damage the system?