After changing my O2 sensors, I developed a leak in one of the hoses to the expansion tank. Is it more likely that the two inch section of rubber hose was already on its way out and started leaking, or that I overtightened the clamp and split the connector?

I am inclined to believe it is the hose. When I first noticed the leak, the clamp (screw type) allowed some play on the hose. I finger tightened it some more. The leak slowed to a crawl, however it is still there as a pinhole leak, but only when the temperature is hot enough.

I hope I don't need a new expansion tank. I replaced it with a new one back in February. Maybe there is also the possibility that it was already leaking through that hose? I recall that a couple weeks ago I noticed my coolant level was a little low. It had been at least a month since I checked it prior.

 

It is wise to check the coolant level in the expansion tank on a weekly basis. As these vehicles continue to age, more and more potential leak/weep spots will become vulnerable....

 

glandaniel, The plastic pipes associated with the expansion tank (I'm referring to the brown tank where you add fluid) are so friggin fragile I'd suspect the plastic 1st, but I admit I can't picture the plastic you are referring to mate. Sounds like you have a real good handle on where the leak is, if not I'd point out how darn deceptive some of them can be. I had a tiny crack on the bottom of my expansion tank for way too long before I even realized it.

Wish I was more help mate.

John