The reason aftermarket "proportioning" valves (that are nothing more than pressure regulators) are inappropriate is that they do not perform the multitude of functions a factory proportioning valve performs. There are several functions the factory prop valve performs that are unique to a disc/drum braking system and very necessary for properly functioning brakes.
One is holdoff. Holdoff prevents brake pressure from being applied to the front brakes till a setpoint pressure level is reached in the rear brake lines. This setpoint is fairly low and the function of the holdoff feature is to allow the shoes to overcome the return springs and just touch the drums when the setpoint is reached. Running larger brakes will change the setpoint value. This allows the shoes and pads to apply simultaneously and prevent accelerated pad wear. An all drum car will not need this feature as all the brakes have the same "lag" to overcome and if the shoes are adjusted properly will all contact the drum simultaneously. An all disc car does not have this lag as the pads "drag" all the time and the application of braking force is "instantaneous" at all four wheels.
Next is true proportioning. Proportioning requires a reference to front brake pressure to adjust rear brake pressure in PROPORTION to front brake pressure. Aftermarket "proportioning" valves do not do this. Why do this? As you know the front brakes to a majority of the braking. On an all drum or an all disc car you can build this in by changing the drum diameter, shoe width, wheel cylinder diameter, caliper bore diameter or pad size. Due to the characteristic differences between a disc brake setup and a self energizing drum brake setup it is simpler to use a proportioning valve to help the marriage of these two systems in the same vehicle. Older, all drum cars used a single outlet master cylinder to supply pressure to all four corners. The needed difference in braking abilities were accounted for by the physical sizes difference between the front and rear brakes. Either larger diameter or larger width front drums were used to balance the braking needs of the vehicle. When discs were added to the front then your pressure requirements changed and the prop valve was invented to accomodate this.
Next is hinge point. This is a rear brake pressure setpoint as well. Once this setpoint is reached the factory proportining valve slows down the RATE OF increase on the rear line pressure. Due to the nature of SELF ENERGIZING drum brakes there comes a point in brake application that the drums will out brake the discs and lock up. The hinge point backs off the rate of increase so that the rears do not lock up before the fronts. For an all drum car hold off is not need as all four sets of shoes will dig in the same amount and the physical size of the front brakes compared to the rear brakes will account for the additional needed braking capacity in the front, this physical differences is what will cause the fronts to lock first in an all drum car. An all disc cars does not concern itself with this as the brakes are not self energizing.
Self energising drum brakes tend to dig in harder for a given input pressure than non-self energising brakes. It's due to the physical design of the system that allows the shoes to partially rotate into the drum and lever themselves in tighter. This requires less brake pressure for a given stopping force than non-self energised drums.
Then there is the brake warning circuit mentioned, it tells you if you lost one side or the other. To reset this I think you need to fix the cause of the pressure loss, bleed and it should auto reset once fixed, if it isn't stuck. You can take the valve apart to clean them. Becareful as it uses O rings to seal things up and parts need to go back together as it was taken apart. The safety circuit also closes of the leaky half of the system to maintain pedal height and retain some stopping ability.
The only appropriate use for these aftermarket "proportioning" valves like the MP or Wilwood ones are in conjunction with a factory valve, not in place of one. The prop vlave setup in a factory application will vary depending on a number of factors and if you look you will see there are several part numbers for a given vehicle that accounts for the various braking options. My 87 Diplomat uses one of two prop valves, The difference is the std duty uses a 10" rear drum, the HD uses an 11" rear drum. Same exact front brakes.
This is applicable to "modern" drum brake cars, modern meaning they have self energizing drum brakes. Older MoPars, like my 38 Plymouth do not have self energising drum brakes and as a lot slower to stop.
The above assumes that the system is properly designed from the factory. This means that the front brakes, be it all drum or all disc, have the additional capacity they need to work. A bodies had a tendancy to lock up the rears on disc applications because the rear wheel cylinders were too large in diameter. Why? I think it's because when they designed the new disc system they didn't take the rears into much consideration and reused the existing B/E body 10" rear drum setup for the 73/up A's. This was too aggressive for the lighter weight of the A body and didn't take into account the different weight distributions of an A vs. the B/E models. Not to mention the different wheelbases changed the dynamic loading of the car, in effect the A body unloaded the rear brakes faster than the B/E models in a hard stop.
A properly setup all disc or all drum brake system will not lock the rears up first. If it does you need to back off the bore diameter of the rear brakes, easiest to do on an all drum setup. Or change the diameter of the rotor/drum (decrease rear or increase front) or the pad/shoe compound (more aggressive up front, less aggressive in back). Then you can use an adjustable valve like the Wilwood to fine tune the system.
On a dual reservior system the safety circuit does more than light up a warning lamp if you lose pressure in one half the system. The shuttle valve that actuates this light also closes off the port on the half that lost pressure. This keeps the pedal off the floor and allows for a reasonable pedal height so the remaining half can still function. There is way more to a factory prop valve than an aftermarket one like the Wilwood/MP piece.