A Bourdon tube pressure gauge is a flattened piece of tubing that makes about a 270 degree arc inside the gauge housing. As pressure is applied the tube tries to straighten out. That motion is transmitted to a geared spindle that the needle is attached to, which turns and makes the needle move. It sounds like the gear has slipped a few teeth. Try taking the gauge out of the dash and see if you can reposition the spindle gear to make the needle read zero again, or close to it. You can use air pressure and an adjustable regulator to test the gauge for accuracy. Yes, the line from the engine to the gauge is a hollow tube, about 1/8" outside diameter.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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