GP'n, not sure how to put the edited replies in, but I'll try to explain as best as I understand it (fully admitting that I'm always learning).
If I understand it correctly, the vacuum booster assists the manual function of the master cylinder by applying vacuum to a diagphram that in turn creates a pressure differential between a system of chambers, which assists pedal movement to apply pressure to the M/C. When the booster fails or vacuum is not present, the brake pedal can travel to the floor, or it can become very stiff, depending on the style of master cylinder and booster used. Most M/C systems have an override that will allow the pedal to be pushed (hard) to the floor and stop the car. This was the exact experience that happened to the Cadillac. This particular car also has seperate proportioning valves to control the rear brakes. One of them had failed, and was spewing brake fluid. My guess is that the system went low on fluid, the customer panic braked one time too many, and that led to the physical failure of the booster (there was an actual tear in the seal where the brake pedal pushrod goes through the firewall into the booster).
Most Jeeps I've seen use a vacuum style booster, although according to my brake book hydraulic boosters were very widely used on GM cars in the 80's. I do not know what style he has. My CJ7 had vacuum.
I don't know why the front drum system is different. I was probably talking through my a$$.
Happens occasionally. It could be in my case that the truck was converted from a manual system to a vacuum assist system, but the drum brakes were never designed for that. My handy Haynes Brake Manual says that drum brake systems do not require the same amount of pedal effort that disc brake systems do because of the "self-energizing action inherent to the design of drum brakes", and therefore power assist was not really needed until the advent of front disc brake systems. Perhaps the power assist is overriding a loose adjuster in the drum, causing the brake to occasionally lose tension. If the wheel cylinder had failed, there would just be no braking on that corner. It shouldn't affect pedal travel all that much, or else there would be big puddles of brake fluid under the wheel.
Hope I addressed your concerns