The OD of two pistons can be the same, while still having different surface areas. Being a fat guy, I will use doughnuts as an example. There aint as much doughnut there when the center is gone.

Think of a hydraulic cylinder. The rod side has less surface area to apply power to the load.

Well, this is interesting.

Let me approach differently. Why would the factory want a different pressure front to back when they can change brake "bias" by changing piston area, rotor diameter, drum diameter, width.

To have different pressure the pistons would have to be hard connected to each other. Maybe some are.

If in the master, the front and back reservoirs and pistons were totally independent of each other, the pedal would stop as soon as say, the rear flow stopped. Once hard in the rear, no flow would go to the front, hence no brakes up there.

Isn't it interesting that the brake system, is far more complex than meets the eye, eh?

Please correct me if I am wrong, I think a lot can be learned here.

Because most trucks have more rear brake than they need when not loaded(talking drum and wheel cyl size)and even with down stream regulation (ride height, rear abs dump etc) rear volume to apply brake shoes can be higher than front discs, while rear pressure can be lower.
As far as m cylinder pistons being connected... that happens through hydraulic pressure.
Another form of bias inside the master can be controlled through the transfer springs. The transfer springs can be installed to which chamber compresses first.

there is a spring between the 2 bores that will
bias the master(on most cars now days)

The old minivans had a prop valve mounted at the rear, attached to the rear axle with a link. As the rear suspension was loaded heavier or lighter, it would adjust the valve which would increase or decrease rear brake bias.

Once you modify your car from OE, bigger or smaller tire, different brake kit, lighter weight or substantially different weight bias, etc., the OE calibrations go out the window. With any major modification and/or replumbing, the OE prop valves, distribution/compensation blocks, etc., hit the can. I don't care if they are a dime a dozen, I won't use an auto part store master if I don't know all about it. Even an oval track and a drag car system and hardware are completely different. Drag race cars need parts designed and configured for drag racing. Just like the parts that make up an engine - if the brake system is a collection of stuff from different manufacturers, picked out of a catalog based on price or bad information, mis-matched, sized wrong, being used for the wrong application, or OE crap being used for something it wasn't intended, it won't work right.

"the front and back reservoirs and pistons were totally independent of each other, the pedal would stop as soon as say, the rear flow stopped. Once hard in the rear, no flow would go to the front, hence no brakes up there."
The reason the rear port goes to the front brakes, they do the majority of the braking, and why discs where used up front. Any decrese of pressure its safer at the back. Everybody knows In the extreme panic stop, a car nose dives, effectively raising the rear almost to the point of no tire contact, with equal pressure, those brakes will lock. This happens with all tires the same size and big fat slicks, tires unload. Granted, center of balance has a lot to do with how the brakes are set up on each type of car. Low light car can be set equal to an extent, normal B-body, you better take some out of the rears and have more up front.

Spring is just used to keep the two pistons separated so brake fluid fills the void and acts as a "liquid" rod, so they move in unison.

Have you ever built a chassis car.. the rear brakes
do most of the work due to size of the rear
tires.. on a chassis car has almost zero dive on
the front

I think we all agree that once we depart from factory specs, brake bias needs to be considered.

I guess I'm just saying I still don't believe you get two pressures out of the same master. From the factory, there would never be an instance that this would be needed.

Now, I understand differential in pressure and how that is obtained. I was pretty good at working on the Navy's Mk 45 gun system. It has some of the most sophisticated hydraulics ever. I get that. Actually, I taught a lot of that.

That mini van adjustment, never knew that. But I did rent many an early 80's Citation front driver GM that had the rear brakes set up for 4 people in the car. When you only had one or two people, it wanted to swap ends.

I will post a pic of a typical pre abs combination valve. Im doing this from an iphone so may not work😀
Most cars built from say ‘72 and later had valves with these features. First, yes, a shuttle valve to lock off a circuit failure, and indicate to the driver that there is a failure (warning light)
Next, the rear proportioning accumulator section.
Also note the front circuit has a spring loaded restrictor. This feature assures the rear circuit starts to react first. Mostly to take up slack in the rear shoe adjustment without shifting the shuttle section of the valve.


It is termed a combination valve because it has 3 functions. To say it is not a proportioning valve at all is a stretch.

As for not believing that a master cylinder can output 2 different pressures, that’s fine with me😂

That is a great picture. Thanks for posting. Learned something today.

As the pic shows, less pressure to the back. The propositioning part came in after disc showed up with their better stoping problems. Now it's anti-lock to keep the wheels turning instead of sliding. Traction control is another variant.
And yes I've built a chassis car and I wanted both close with a tad more bias to the front. Most any brakes are fine till you lock them, then you find out to late how well they work.

I've never had the brakes lock up after I got them adjusted
but you can do what ever you want

Unless they are heavy (2600+), most of the tube cars I've seen or dealt with have two 4-1.75" piston calipers on the rear and two 2-piston or even single piston calipers on the front. Basically just enough brake to be able to do a burnout. With 14" tires on the rear and 3.5 or 4" on the front, having too much front brake is asking for trouble. A lot of Super Stockers and even Stockers are going to 4 calipers on the rear so they can use 4 piston calipers on the front and race the stripe and/or get the car stopped without sliding the front tires.

Something to add to the mix here.
So on a street/strip car with a 2x3 tube chassis with 14 point cage running 7.5" x26" tire up front and 33" x 19.5" or 22" rears, 4 wheel Wilwood discs. What would be the optimum bias setting?