Heck, I went downhill in a suburban and it was hairy enough- I can’t imagine driving it with 80,000 truck and load. Scary is an understatement lol.
I've driven those mountain passes numerous times. I've had to pull over on occasion and let the brakes cool off. I could see smoke coming off the trailer wheels. If that truck got over about 30 miles an hour and you had to stop you had a problem. They don't slow down fast on flat ground.
Hitting the tractor brakes hard could cause the trailer to jackknife. They have runaway gravel truck ramps for rigs that can't stop. Never had to use one but I've seen those that did.