http://www.northerntool.com/shop/tools/product_200329825_200329825

We don't have one, but many folks do.  They just feed them from the 50 psi system pressure, and the pump boosts it to 150+ for the washdown hose.  You turn the pump on washdown, and cut it off when you're done.  In your case, I'd suggest torching a hole in the top of the old water heater, and mounting a slow-filling watertrough float-valve to refill the tank between milkings.  Plumb a booster pump between the bottom of the water heater and your washdown hose. 80 gallons should be plenty for a typical washdown, and you won't be fighting with the wash vat for water pressure.