What is the proper response if your brakes fail on a downgrade?

If your service brakes fail on a downgrade, the priority is to bring the vehicle to a controlled stop using whatever resources remain. First, downshift to use engine braking. If your route has a runaway truck escape ramp, use it without hesitation — these are loose gravel or sand ramps engineered specifically to stop runaway commercial vehicles safely, and there is no penalty or shame in using one. If no escape ramp is available, look for an uphill grade you can use to bleed off speed, or use a long open stretch of shoulder to scrub speed against guardrails or embankments as a last resort. Setting the parking brake at full speed can lock the rear wheels and cause a jackknife. Pumping brakes might restore pressure briefly on a hydraulic system, but air brake failure is rarely restored that way. Plan for brake failure before it happens by knowing where the escape ramps are on your route.