Just take it out and keep adding coolant till it comes out there. Put the bolt back in then run it till its warmed up and then loosen it and make sure coolant is coming out and your system will be bled!
My truck has developed a flare/slip on the 4-5 shift especially bad when WOT on the big tune. Any ideas as to what's on the way out? It has a PPE stage 4 trans kit that was installed around 3 years and 40k ago so I don't think it owes me anything.