Gauge staying "up": could be in the gauge itself. Maybe disconnect the tubing to test? If there's no pressure when not running, probably the gauge. If the needle drops when you disconnect the tubing, mebbe something upstream.
Copper vs Nylon tubing: For what it's worth, back in the olden days('60's & '70's), everyone recommended *steel* tubing for mechanical oil pressure gauges. The claimed reason was that copper would fatigue & break. About 34 years ago, I was riding in a car when a copper OPG line broke- made one %#@&* of a mess!
Folks more car-savvy than me said breaking *inside* the cabin was unusual- they said it usually happened close to the block. Nylon line usually needs the little steel inserts on the ends to seal completely so the ferule doesn't crush the tubing. I'd prefer that to copper, but not by very much, & it may be tough to find those little inserts.
If it was my car, & I *had* to run a mechanical OP gauge, I'd look for some steel tubing. Copper line worries me a lot. Even better, IMO, would be a good modern electrical gauge. Accurate enough, & it'll *never* hose down your cabin with hot motor oil.