During the pre-ABS era, manufactures did most all they can to prevent rear brake lock-up in most every road and vehicle load condition possible.
They knew well if the rear brakes were to lock-up, the risk of a spin under braking goes up a lot resulting in a very un-safe road condition.
One other factor is brake modulation which was important back then as it gave the driver the ability to lock-un-lock the brakes as required to lower stopping distances. Most all of this has gone as ABS did way with the need for good pedal feel and ability for the driver to modulate the brakes since the ABS system will do that automatically. Once this design concept stuck, work was done to lower pedal effort as there was little reason to have high effort at the brake pedal since the ABS system will prevent brake lock-up.. why have high brake pedal effort? It was not necessary in a brake system that basically cannot lock up the brakes and the system will allow steering under braking even if the car was in a slide.
This is why the brakes on the exxe is so very different than brakes on a modern car with power assist and ABS. They are essentially from different planets and places in time. It is also why the exxe has brakes that are biased towards the front and less in the rear, done to prevent rear wheel lock up under hard braking. In the extreme case, the Lancia Scorpion-MonteCarlo has power assisted front brakes with no power assist in the rear. This is an example of extreme front brake bias which resulted in a number of other problems including a reputation for poor braking performance. The brakes on the Scorpion-MonteCarlo was one of the sources of major grief that gave this design a ever-lasting reputation for being AWFUL beyond being under powered.
Braking performance in the exxe can be improved by increasing the brake caliper piston diameter in the rear thus increasing the amount of braking force in the rear of the chassis. This increases the risk of rear brake lock-up at the trade-off of lowering stopping distances by making all four brake calipers work in better sync. I don't believe this alteration is for every driver, road condition and more. It is a performance related modification that will increase the risk of rear brake lock up.. In ways, it is much like reducing the amount of understeer in the chassis. Cornering speeds and control goes up at the trade-off of less benign chassis behavior near or at the tire adhesion limits.
Brake pressure limiting valves work by limiting brake fluid pressure once that set limit is reached. Below that the valve does little to nothing. This is why the manufactures of these valves does not recommend these valves being used on the front brakes as it would limit front brake stopping power.
Using two independent brake master cylinders with a "Bias Bar" results in a linear brake pressure adjustment since the change in force remains much the same over the adjustment range due to essentially moving the brake pedal's point of applied force, not limiting brake fluid pressure.
Brake setups that are designed for FWD or front engine-rear wheel drive cars often do not directly transplant to a mid-engine chassis due to chassis weight bias or where the weight is located. FWD chassis often have much larger brake caliper pistons compared to the rear while a mid-engine chassis can have nearly the same brake caliper piston diameters front and rear.
Bernice
Bernice got it right. Brake performance is ALL about balance. Our X's are typically weight biased to the rear. That means you can maximize front brake performance, then still have the option of adding more stopping power for the rear, than a front heavy car will have. That equals more overall brake performance is possible for a rear heavy car. If you maximize front perfomance, without maximizing rear performance, you leave a little on the table, but it's safe.
However to maximize brake performance you HAVE to get the bias right. And, it HAS to be right for the road conditions you're driving in. If you have your brake bias set for ultimate dry performance, and drive in the rain, you'll find the rears will lock up on you before the fronts and - wooops - the rear of the car tries to pass the front. That's why easy brake bias adjustment is important.