What is the proper procedure if your brakes start to fail on a downgrade?

If your brakes start to fade or fail on a downgrade, look for a runaway truck escape ramp and use it without hesitation. Escape ramps are loose-gravel or sand inclines specifically engineered to stop a runaway truck safely. The longer you wait to commit, the faster you will be going, and the harder it becomes to bring the truck under control. Pumping brakes does not restore an air system the way it might (briefly) help a hydraulic system. The parking brake at speed can lock the rear wheels and cause a jackknife. Driving onto the shoulder at high speed risks a rollover. If no escape ramp exists, use an uphill grade if available, then a long open shoulder if that fails, then guardrails or embankments as a last-resort speed-shedding option. Knowing where the escape ramps are on your route is a critical pre-trip planning step.