Safety Brakes.
After HORRENDOUS crashes from brake failures, safety regulation bodies demanded redundancy in brake systems...
This will depend on what YEAR model you have, manufacturers have to be beat along the path of safety, they rarely do things that increase cost on their own...
The earliest systems were single cavity master cylinders that fed all brake cylinders.
One leak, bad pump in master cylinder, you lost all brakes at once.
When I work on one of these that's not a display piece or hanger queen,
If it's going to be driven, I update to safety brake master cylinders so the front & rear are on different lines.
I'm all about proper restoration for museums, but highway safety is something I won't compromise, if it's going to be driven often on public highways, it MUST be updated.
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'Safety Brakes' have two separate brake systems.
Two separate hydraulic fluid cavities in the master cylinder, to different hydraulic pumps (on common shaft) in the master cylinder, two separate feed lines to the safety valve.
In drum brakes, this pretty well split the system front/rear,
The safety valve will shut down the end with the leak, giving you REDUCED braking capacity, but you still have brakes!
With disk brakes you will have two different sized bores for the plungers/pistons in the master cylinder since disks require different volumes/pressures than drums.
The common plunger/piston rod makes this an elegant design.
Simply two sizes of bores, two plungers on one shaft, and this works quite well with few failures.
Now, if you have the newer style 'Safety Brakes', (disk brakes)
These realize the fact about 70%-80% of braking is done on the FRONT brakes (weight transfers forward when braking, increasing traction),
So you will find there are lines to EACH front brake,
While you will find the rear brakes are on one common line which services both rear brakes evenly.
You will see two FEED lines from master cylinder (front/rear) to safety combination valve, and THREE lines coming out of the combination valve.
Again, the idea is to give you as much braking power as possible in the event of a failure at one corner, the two front delivering more braking power than the rear combined.
The safety valve simply uses high pressure on one end to push the valve off center to block the low pressure line(s) that have leaks.
This saves fluid (which is a fire hazard) and let's you know there is a problem by activating the 'Brake' light on the dash.
When you bleed the rear, it's a common line, doesn't matter which side is open, they are both on a common line.
When you open a front wheel cylinder, swap back and forth Left to Right and back to left, etc.
The fronts have separate lines in most newer CJs, keep that pressure consistent so back pressure doesn't build up and move the spool.
Again, there are very stiff springs in the valve, an open valve, front/rear alternation and LIGHT pressure on the pedal will often keep the valve from moving.
(Ever wonder why professionals like to bleed brakes on a lift so it's easy to move left/right, front/rear?)
Once you have a safety combo valve apart and understand how it works,
It's design genius becomes apparent, it's VERY elegant and durable design...
But it DOES require clean fluid, no moisture.
Dirty fluid or anything that will cause corrosion can stop the valve from working correctly, I've pulled them apart and found them caked solid with crud/corrosion,
So what the OP is doing by changing/flushing the hydraulic system is a REALLY good idea.
When clean, the system lasts for decades.
(how often do you change master cylinders or safety valves/wheel cylinders?)
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This by no means is the last word on brakes,
We didn't cover hydraulic differential ratios, things like back pressure valves, a ton of other stuff built into brake systems you shouldn't ever have to change when you simply replace failed components or do maintenance.
I'm NOT encouraging anyone to attempt to re-engineer your brake systems,
That takes a specific engineering education to be even remotely safe, even more difficult to be redundant like the factory system is.
Just switching from drums to disks can be challenging, or from manual to power brakes can be challenging.
The backpressure valves in the bottom of the master cylinder usually drives drum to disk conversion guys nuts...
