I simultaneously loss my power steering and power brakes.
I have a 2003 GMC crew cab with the Duramax diesel. I started the truck in the drive and noticed I couldn't turn the steering. Thinking that the wheels may be obstructed by something I put the truck into drive to move it a few feet forward to see if that fix the problem and that's when I noticed that the brakes would just barely stop the truck's forward motion.
I had the truck towed in to my GMC dealer for repair. The technician at the dealer said that on my year truck the power brakes are hydraulic assist and the assist comes from the power steering pump. Apparently there is a valve in the pump that when it becomes obstructed you loose both power steering and power brakes. The recommended fix was to flush the power steering system and replace the hydraulic fluid. I'm not convinced that's the correct fix.
I've notified GM and the NHTSA via letter of this incidence.
Can this problem happen while you're driving down the road? I'm going on the assumption that it can, so take care.