I replaced the oil pressure gauge on my 1941 Case SC and in doing so had to replace the oil tube leading from it to the engine block so the fittings would work. The new tube is clear plastic and I noticed that the oil flows up the tube about 5" and not any farther (the tube is about 18" long). will this affect the readings? I wondered whether I needed to fill the tube with oil (like priming a pump) to get an accurate reading? Right now it only shows about 30 pounds of pressure that stays constant after an hour of running.