If a truck driver loses brakes going downhill the truck will accelerate very rapidly as a result of the gravity and potentially lead to a serious accident.
If the truck driver is able to use their Jake brakes that can help to slow down the truck and help them safely stop the truck.
Or if possible the truck driver may be able to apply the clutch and put the truck into the reverse gear and slowly let up on the clutch to help bring the truck to a stop going downhill.
My dad was a truck driver and he once did lose his brakes going downhill and he didn't have Jake Brakes on his truck but he instead applied the clutch and then put the gear into reverse on the manual transmission and then slowly released the clutch which thankfully brought the truck to a stop.
The best way to descend a slick grade is to select a lower gear, brake and power up at the same time/power braking.
Semi trucks use air pressure for the brakes and so if a truck driver overuses the brakes, air is drained from the compressor faster than the compressor can replenish it.
As a result the semi trucks brakes then have no air pressure to feed them.
You now have a runaway truck.
This is a problem especially on long descents downhill.