It's just a safety thing....
For the 1967 model year, the federal government mandated dual braking system via dual-reservoir master cylinder for vehicles. But, I do know a 1980 2-1/2 ton GMC trucks were using a slave cylinder that pumped into a hydro-vac unit. If slave cylinder failed, no brakes.
If one side of an older dual braking system failed, vehicle will have braking ability, but if extreme brake pressure is applied, vehicle may spin.
Somewhere in 1980s, GM was using diagonal-based brake line system, so if one side failed, then opposite front and rear shoes/discs would would working. Like left front and right rear.
When a brake system is maintained, single or dual reservoir master cylinder, I suspect there were few failures. With a single reservoir master cylinder (or slave cylinder), I suspect there was a warning sign of impeding failure in many failures, like a lower brake pedal position before brakes were applied. If a brake line busted, which was common many years ago, then a complete failure.
Brake Line History
A little history- Before WWII, many manufacturers used copper brake lines. Copper is both easy to form, and resists corrosion well. Unfortunately, copper is also prone to cracking. Shortly before the war, there was a transition to steel tubing for brake lines. Steel did not have the cracking problem of copper, but it did corrode. Steel brake lines would often fail after only a few years of use. The options to prolong the life of steel brake lines included many types of coatings to protect against corrosion.
Steel brake lines failing due to corrosion was not acceptable to safety obsessed Sweden. Both Saab and Volvo experimented with epoxy coatings, anodized steel and various other coatings to protect the brake lines. Volvo introduced a new type of brake line in 1976. This new brake line consisted of 89% Copper, 1% Iron, and 10% Nickel. This alloy proved to be durable with the corrosion resistance of copper and the crack resistance of steel.