If your brakes suddenly fail at speed, what should you do first?

If brakes fail suddenly, the priority is to slow the vehicle using whatever means you have. First, pump the brake pedal — on hydraulic systems this can sometimes restore enough pressure to slow the vehicle (it does not work for air systems). Then downshift to use engine braking, which slows the vehicle especially well in lower gears. Look for a safe place to steer to: uphill terrain, a soft shoulder, or a long flat stretch where you can coast to a stop. As a last resort, scrape the side of the vehicle along a guardrail to bleed off speed. Pull the parking brake gradually, not abruptly (an abrupt pull at speed can lock the rear wheels and cause a spin). Turning off the engine eliminates power steering and power brakes (in some vehicles) and is generally a bad idea while moving.