Slow gauge

those oil preasure lines are curved brass tubes in the gauge that "unwrap" under pressure and are connected by a small gear that acts on the needle. if you can take the feed off the gauge and allow it to blow into a cup you can see if it just dribbles out or jets out. if it dribbles out, suspect a kink in copper line. if it jets, then maybe the gauge mechanism is rubbing on something. you can manually touch the sense tube in there and the needle should move with little effort but dont firce it as the tube will bend and the zero will be lost