The brakes on a FedEx truck, as with most of the trucks in use in the US today, are "S-cam" brakes. This is a picture of the inside of one:
This sits inside a cast-iron drum, to which the wheel is bolted. The black thing that looks like a fat letter S is connected to a brake chamber through a "slack adjuster." There are two kinds of brake chambers on a truck.
The first kind is on the steer axle. In it is an inflatable rubber bag. When the driver steps on the brake pedal, this bag inflates. It pushes on a rod, which is hooked to the slack adjuster, which rotates the S-cam, pressing the brake shoes against the drum and slowing the truck down. All trucks have two of these.
On all the other axles on your truck the chamber's different: there are two bags and a huge spring. The spring shoves the shoes up against the drum and stops your ass in a right hurry. (This spring is made from steel rod 1/2" in diameter. There's nothing subtle about it.) One of the bags is your "emergency brake." Its purpose is to compress the spring, lifting the shoes away from the drum so the truck will move. The other bag is there to compress the emergency brake bag when you step on the brake pedal, which allows the spring to press the shoes against the drum and, as you expected, slowing the truck down. On a five-axle truck there are eight of them; FedEx pulls doubles so there are ten of these on one of their rigs.
If the fire burned through the air lines, all the bags would deflate and the spring brakes would take charge.
Now here's your problem: DOT tubing is nylon - probably for this very reason; if your truck catches fire going down the freeway they WANT the brakes to come on all by themselves. The fire consumed the lines, but you'll never know when...but I don't think it was before the crash.
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