I have a sneaky feeling it was the Booster all this time. Went back to my receipts and found I purchased a crappy Autozone Unit. I replaced with 2011 ACD but didn't get a chance to confirm the upgrade worked as my S-Box sprung a leak instantly. Actually found a place that rebuilds OEM units with GM parts. Fingers crossed, but it sure felt better. Brake pedal was up high like normal. Haven't had that in a while. This started about the same time I had a Intermittent Short on my 3-axel TH. No trailer brakes, no truck brakes. Took a bunch of trouble shooting to find trailer brake problem. Ended up being the Solenoid, it would short out when it got hot. I could not make it do while sitting on the driveway. It's nice to tow with Truck and Trailer brakes again.Glad your issue is resolved.
Thanks for giving us an update!