Thanks! I don't believe it's the breather as it just sits for now while we're doing some work to it. It hasn't been driven more than out of the garage and onto the driveway. When I get home I'll pull the tube from the housing and replace the o-ring.
I'm thinking a coolant line shouldn't be leaking as the are up too high for a sitting truck to leak. If it had been driven, I look at the lines and breather too. I just wasn't sure if there was an o-ring on the tube, but I thought there might be.
Thanks again.