Further to my earlier post about the testing that was performed in the USA and the requirement for brake proportioning, I dug up the report to remind myself what it said, and see that I misremembered it.
Two cars were used for testing, #1006 and #1011. #1006 was tested first, in late August/early September 1970, and the tests were conducted at Riverside Raceway, with ambient temps in the 90s. Following the successful conclusion of the tests, they went to repeat them on #1011 but found excessive pedal effort was required. They suspended the tests, and according to the documents, eventually they swapped the master cylinder *and the proportioning valve* over from #1006 to #1011. That restored normal braking operation, and the tests continued with #1011, which more-or-less mirrored the results earlier attained with #1006. The tests on the now-functional #1011 took place in mid-September 1970.
So, clearly the cars had the proportioning valve from the start. Where earlier I said that I'd read that it had to be added to the car to make it stop properly, now I see that it had to be transferred from one car to the other; presumably the second car had one installed originally and it was just replaced, not added. So therefore, for better or for worse, it was part of the original design.
It's really interesting to see how aggressive and comprehensive the brake tests were, and how well the car performed. It's also interesting to note that at the extreme limit, on one test one of the cars experienced lockup of the left REAR wheel. That shows that bias was fairly evenly distributed between the front and rear, when the cars were new and the tires were skinny.
It was interesting to see that they were not trying to achieve (nor did they measure) stopping distance; rather they sought to achieve a fixed deceleration rate, measured in feet per second per second. They measured pedal effort to achieve that rate on multiple stops, to measure brake fade--if it took more pressure to achieve a given rate of deceleration, then that indicated that the brakes were fading. They also did post-fade testing, to make sure the brakes recovered once they had cooled, and they did water testing--driving the car through 9 inches of standing water to soak the system, then honking on the brakes etc.
Rotor temperatures were measured, and as expected with solid rotors, they got quite hot--up to 600 degrees after 15 consecutive 60-0 stops with no cooling in between. However, brake pedal effort to achieve the desired fixed deceleration rate actually remained about the same (the effort went down as the brakes heated up, then went back up as they heated up further, so that the effort required on the last stop was about the same as that required on the first stop).
Although they didn't report the actual stopping distance, if you're starting from a fixed speed and slowing at a fixed rate, time and again, the stopping distance must be the same (plus or minus a bit). So the car passed the brake fade and recovery tests with flying colors.
They also tested braking with each system (front and rear) disconnected; there they did measure braking distance. The government requirement was a stop from 60 mph in 646 feet; with the front disconnected it took 378 feet, rear disconnected it was 284.6 feet.
They also checked for performance with the power assist disconnected, from 60 mph; the requirement was a stop in 600 feet, and the two cars did it in 319.5 and 324.2 feet respectively.
The report on the braking system tests (oh yeah, I forgot to mention they also tested the effectiveness of the emergency brake) is over 100 pages long.
The bottom line is that when the car was new, it had GREAT brakes relative to your average 1971 car, and their performance would certainly be more than acceptable by today's standards (for normal street use). If a stock Pantera today doesn't have great brakes for street driving, something is WRONG and needs to be fixed.
That said, they certainly can be improved upon, and there is never any harm in doing so!