In your cooling system, you need a 50/50 cooolant/water. And in your "coolant" res, you need straight coolant to the levels indicated on the side of the res. When the engine gets hot enough to create pressure enough to overcome the pressure specified on the red cap (I think it's 16 or 18 lbs.) water/coolant flows into the res through a small rubber hose from the base of the rad cap, to relieve the extra pressure, mixing with the pure coolant that should already be in there. When the engine cools down or is turned off, the system creats a vaccume that sucks coolant from the res back into the radiator, thus the importance to always making sure you have coolant in the res bottle, because you don't want air getting sucked into the system, since that can cause problems fast. You don't need to top off the res but just to the levels shown on the bottle according to if the engine is cold or warm/hot. There is an overflow on top of the res bottle in case in the event of extreme overheating, it allows the radiator mixture to escape without blowing something up, granted something is not really wrong. If you find you are filling the res bottle often, have your jeep checked for a blown head gasket or something related, since yours is not like mine, a 2000. Then I would just say a cracked head. But no cracks on mine so far, at 118,000+ miles.
I hoped that helped you.
