The spring in the cap keeps the rad pressurized. When the coolant gets hot it expands and pushes up the spring in the cap and runs out to the overflow, then is drawn back in when the coolant cools off and the spring returns to normal. So the rad is pressurized but the overflow isn't and doesn't need venting. But the tube has to run to the bottom of the overflow so air doesn't get sucked in.
Now, 84 CJ7 Laredo , weber 38, 3.73's, Alloy 1pc., AuburnGears LS, hei/tfi, 2.5" BDS, Perf. Access. Years ago, 81 CJ7 304, 33's