02 - you can do it without filling the hose, but I recommend it. At first I did just fill the isolator and the gauge worked, but I never got as high of readings as I should have, and the needle moved very slowly. Now that the line and isolator are full, the gauge moves a LOT faster and is more accurate. I filled the isolator, and once I started the truck the coolant moves up the line toward the gauge. At that point I disconnected the tubing, and filled the isolator again, which did the trick. You guys will laugh but I actually used a straw to fill the isolator... dip it in the antifreeze, plug the top of the straw, and move it over, lol. .