Both my '67 and '72 had failing regulators when I got them. They passed water, just wouldn't regulate.
I would also suspect a plugged screen in the city water hookup at the rear of the rig. The '72 has a check valve back there as well. I'd inspect the connection point back there first.
BTW I always hook up city/park water with a cheap inline regulator and filter. With all the plumbing issues associated with vintage trailers it's 1 less thing to worry about.
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