Finally got it - something I overlooked from the beginning and didn't suspect.
When the 7-pin connector on the end of the pigtail was plugged into the female plug on the back of the truck, it wasn't making a connection even though the connector was locked onto the plug. Even when I push as hard as I can, it barely makes a connection on the brake circuit. So now I just need to get a new plug for the truck.
I also found one of the tabs inside the end of the pigtail connector was spread apart a bit and and I bent it back to where it should be. It was for the power to the brakes so that didn't help either. Might get a whole new pigtail & connector too
Once I get the new plug on the truck, I can do another road test and make sure it's all good. No big surprise, our local GM dealer wants 4 times the average cost on the internet for a new GM plug.
