When I replaced all of the copper in our '74 Argosy 26' I totally eliminated all of the drains and interior pressure regulator.
I use air to blow out the lines, then pump RV antifreeze into the system. We live where it gets to -40 in the winter. Have not had any problems with freezing in over 5 years of use.
I believe in the KISS rule; Keep it simple stupid. The fewer devices in the internal plumbing system the better. Pressure reliefs and regulators fail. When they are inside the coach they are much more difficult to replace.
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