We have not seen a 3x1regular oring blow yet or the machining on any of the ports of the 3x1 as of yet. But its poss that the oring was pinched. You stated it was twords the very back which is the 8x1. I have seen these blow if manifold wasnt installed and tightened all the way. My next oring was for you to replace was the 8x1weight as it's happened a few times. The 3x1oring would not leak heavily if it was pinched. I dought it wasn't something missed during build as all markers are tested.
My prognosis would be the 8x1.
If you cant find one or thr 3x1 please let us know.
Please email me directly.