I don't subscribe to the theory that if it usually breaks, it's probably broken. I find out what and why before replacing parts. Perhaps that comes with 20+ years in IT...I work on site and customers don't appreciate guesswork.

When they opened the front bleeder valve with the rear system pressurized the shuttle moved to the front. My guess is that the shuttle return spring is broken or corroded, or that the distribution valve accumulated a layer of "mud" that the shuttle stuck into. In either case, the shuttle did not return to the center position and the fluid from the reservoir could not build pressure on the FRONT of the shuttle to recenter it. If you look at the brake line diagrams posted on my site, or under your hood, you will see that the front port on the distribution block goes to a front brake line, not to the master. The master feeds the front driver's side port on the distribution block. Without pressure on the front of the shuttle and with high pressure against the rear of the shuttle, and with suspected internal interference, the shuttle was not going to return to center. The pedal was low because there was no pressure to the front wheel cylinders.

If, however, they had opened a rear brake bleeder and removed the line from the front port of the distribution block and used a very narrow pin punch to recenter the shuttle, there is a good chance it would have been able to be returned to service.

If the line from the master cylinder was routed to the front of the distribution block rather than the side, I do not believe this condition would occur as the pressure from the master cylinder would serve to push the shuttle back into place.

And as to that 1% when the brakes fail totally...the majority of cars on the road today are maintenance-deferred or neglected. I see it every day, even on newer cars. Since we don't have safety inspections, how many cars do you see every day with bad shocks or struts, hanging exhaust systems, squealing brakes, burned out headlights, or burned out brake or tail lights? It's not just older cars. In this case, though not intentional neglect, failure to flush the brake system on a periodic basis likely contributed to the situation and compounded the problem. Hydraulic systems are not designed to be maintenance free for 35+ years.