If you're forced to ride the brakes to keep your speed in check, I'd be inclined (pun intended!) to see whether or not you're within the engine's fuel cut-off zone first.
I've seen vac tanks added to EVs - they don't have to be as giant as a propane tank. People have used DIY sections of capped PVC pipe plumbed into the system.
Lastly, bryn, you should add your car to the garage so we know what you're talking about (it'll appear under your username in every post):
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