One question for all you guys saying it's air in the system.....how did it get there after 1.5 years (roughly)? I'd assume (and maybe I'm being naive) that *eventually*, any air in the system is going to work it's way back up to the reservoir and out through the overflow tube. Is that not the case?
The reason there are bleeders in cooling systems at certain points is because air or vapor can get trapped at a local high point (like an upside down U in a line that continues below that point)... the result is that the system may not self-bleed there, so you have to do it manually.
Assuming you started with a properly bled system, one way to get air into the system would be to start the engine, get it very hot, then shut it down and let it heat soak (sit with no further active cooling). Coolant sitting in the very hot head could boil, turn to vapor and push coolant into the overflow tank. If you remove the radiator cap before things cool down sufficiently, atmospheric pressure is insufficient to collapse the area filled with steam back to liquid (it could end up sucking air through the open radiator cap instead of coolant from the expansion tank). On the next startup, you have a void in the cooling system not filled with coolant. This lets the head get even hotter on the next start up, creates more steam, dumps more coolant into the expansion tank and potentially even out an overflow if the expansion is sufficient.
I’m sure there are other ways for this to happen. For example, the constituents of coolant break down into their component gasses (hydrogen, oxygen, etc.) when heated many times. Expansion tanks and swirl pots are usually sufficient to remove the resulting small gas bubbles in the system. But if a cooling system were operated for short periods, it is conceivable the normal course of events - the bubbles get removed - could be circumvented. The bubbles would then migrate to high points in the cooling system and create voids in the coolant. The chemical reaction of coolant with metals can also create gasses. For the record, although this mechanism is possible, it seems less likely to me.