Here is the best picture of a "proportioning valve" I have seen. This is the valve necessary to give you the dual brake system "cut off the leaking side" brakes.

That is not a proportioning vale. It is a combination valve. One of it's functions is proportioning.

The cut off the leaky side function isn't it's main purpose.

It's main purpose is to mitigate the functional differences between disc and drum brakes.

Note that I put "proportional" in quotes.

How does it differentiate there?

Missed the intent of the quotes, lol, sorry.

Not sure what your question is though

it differentiates in that is serves two purposes.

1. Being a cut off valve as mentioned.
2. Providing a "PRESET" amount of proportioning or delay to the rear brakes.

There are valves that do one or the other A. bypass valve for isolating the leaking side. B. Proportioning valve for delaying application of the rear brakes.

The pictured valve does both, hence the term "Combination valve". These types of valves are typically non- adjustable so one has to be careful when selecting them as they may or may not work as desired in a modified braking system depending on a bunch of other variables.

Wrong.

A dual circuit master cylinder is what's necessary. Any other valves are for fine tuning. There's numerous OEM applications that are plumbed with the front brakes straight to the master and rear brakes may have a proportioning valve or an ABS valve.

Go to your car without a valve and open a bleeder. The pedal is going to go to the floor. Period. I wanted to stir things up, which I why I posted.

But, I doubt I can change your mind. Last summer, two different tracks, two "dual" masters, brake leak and pedal goes to floor because, no valve. Both mopar type masters.

Also, there is no delay in rear brake apply. The proportion part is a set pressure regulator limiting pressure to rear to prevent lockup.

I'm done, have a nice day.

Is this a race car?

Street car, the brake warning light may need to be functional and all the parts there to pass inspection. Otherwise, it's not used on race cars. Not much chance of rusted brake lines or old hoses blowing out.

Design the system correctly, use the right parts, plumb it correctly, and you likely won't need any kind of band aid valve.

There most assured is a delay, but it is to the discs. It's called hold off.

Then there is hinge point where the rate of pressure increase to the drums is slowed due to the self energizing function of modern drum brakes.

And the aforementioned proportioning.

What trucks are you talking about?

I've been in plenty of vehicles with blown brake lines here in Ohio. So, I've played your game. Yes the pedal has extra travel but, one half of the brake system still works.

So here's a challenge to you. Disconnect a line at the master and plug the port. Your pedal may feel ok, but you're still only operating half the brakes.