Don't you hate when something like this happens?
My family and I were 30 minutes up a mountain trail today when we stopped to look at the view. I noticed a bubbling sound, and discovered it was my coolant in the reserve tank, boiling.
Now do we continue, taking the chance that we'll overheat farther up the mountain and be stuck? No, we turned around back to civilization instead, keeping my family safe.
Got a new radiator cap on the way back, and hope this solves the issue.