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Concerning the tool, I have used a very large crescent wrench on mine. I needed to take the overpressure relief valves off in order to use it though. I then put cardboard on each side of the valve to keep from scaring the valve's finish. Looking at your, I think they may be different than newer twin tank manifolds, as it seems to have only one overpressure relief valve. Also, it is probably the old type with the lead insert that is pushed out in an overpressure situation. That should not matter (having only one), as it is all in the same system, pressure-wise. The overpressure relief valve will need to be replaced with the modern one (copper disk) in order to get you system working.
SeaRat