Ah, why didn't I consider the Explorer? Smaller wheel bolt circle, four-wheel disc brakes, 8.8" rear axle!
Comparing master cylinders on the basic TJ Wrangler (disc front/drum rear brakes) versus a TJ Wrangler Rubicon (four-wheel disc brakes), I picked 2005 model year as an example. Both brake systems use the same master cylinder that year:
1 04798157 1 MASTER CYLINDER, Brake
R4798157 Mopar Remanufactured Part
When you get to the proportioning valve, there are two part numbers for the 2005 Wranglers. These two numbers apply to all models of the Wrangler, both disc front/drum rear and four-wheel disc brakes. The proportioning is the same part number for each of these late systems. (I describe the function of the proportioning valve in an earlier post at this topic, and you can see why the same valve works for both systems.)
The Mopar part number distinction for the proportioning valve is not whether the vehicle has disc front/drum rear or four-wheel disc brakes. One valve is for ABS models, the other valve is for non-ABS models:
05083807AA [BGK] (anti-lock brakes)
05083808AA [BGA,BRW] (without ABS)
As for master cylinder bores, your stock 1986 CJ-7 master cylinder for power brakes can be either 1" diameter or 15/16" diameter, depending upon the cylinder manufacturer and vendor. The Ford Explorer OEM master cylinder should have a 1-1/16" bore.
The rear/drum brake reservoir and port is at the front of your CJ-7 master cylinder. The bigger reservoir, closer to the firewall, is the disc front brake portion of the master cylinder. Here is a photo with a nice description from one aftermarket master cylinder supplier:
Master Cylinder 1986 Jeep CJ-7 with Power Booster.jpg 29.83KB
This is a typical aftermarket replacement master cylinder for a 1986 Jeep CJ-7 4WD model with power booster. The bore is 15/16", although some designs for this application are a 1" bore size.
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I researched bore sizing to determine how your Jeep CJ-7 brake master cylinder pressurizes the brake system then compared that with a 1996 Ford Explorer master cylinder—specifically to determine the braking system pressure that reaches the Ford Explorer rear disc brake calipers.
The brake pedal leverage and power booster determine the mechanical pressure pushing against the master cylinder pistons. What we want to know here is the fluid pressure that the master cylinder delivers to the rear disc brake calipers. This is determined by the area of the master cylinder bore size.
Let's assume that your CJ-7 Jeep cylinder is the smaller 15/16". The formula for the area of the bore size would be Pi times the radius squared:
15/16" = 0.9375" bore diameter
15/32 = 0.46875" radius of bore
Pi = 3.14159265359
3.1416 (rounded up Pi) x 0.46875 x 0.46875 = 0.69 square inch
For the Ford Explorer master cylinder area:
1-1/16" = 1.0625" bore diameter
17/32" = 0.53125" bore radius
Pi = 3.14159265359
3.1416 (rounded up Pi) x 0.53125 x 0.53125 = 0.8866 square inch
Expressed as pounds per square inch fluid pressure coming out of the master cylinder, let's apply 750 pounds of mechanical force, using the brake pedal leverage and power booster:
Jeep CJ-7 master cylinder fluid pressure into the system = 750 divided by .69 = 1087 PSI
Ford Explorer master cylinder fluid pressure into the system = 750 divided by .8866 = 846 PSI
With the same mechanical force of 750 pounds applied to the master cylinder pistons, the CJ-7 cylinder will pressurize the brake system at 1087 PSI, and the Ford Explorer master cylinder would pressurize the brake system at 846 PSI. This is the fluid PSI pressure going to the front and rear brake calipers. At that stage, the actual apply pressure of the caliper pistons against the rotor faces is determined by the bore size of the caliper pistons.
The actual pedal pressure/leverage point and brake booster in this case is stock CJ-7. The difference in master cylinder bore sizes has the Ford Explorer rear disc brakes getting more apply pressure with your CJ-7 Jeep master cylinder than the same brakes would get on a Ford Explorer. The end result is more brake pressure at the retrofit rear calipers in your CJ-7 than the brake pressure in a Ford Explorer—applying the same mechanical force (pedal and booster) at the master cylinder.
You should notice some difference with these rear discs over the OEM Jeep CJ-7 drum brakes. From what you share, the good news is that you're apparently not over-powering the rear brakes, causing rear wheel lock up or running the risk of spinning the Jeep out on a slick surface. Your CJ-7's beefy Scout II front axle, 35" tires, stiffer suspension, plus the shorter wheelbase and shorter overall length than a Ford Explorer, also contribute to the brake system's balance.
The difference in master cylinder apply pressure is not dramatic, even less if your CJ-7 master cylinder is actually a 1" bore and not 15/16". Your use of the late Jeep TJ Wrangler Rubicon four-wheel disc brake proportioning valve also helps keep brakes in balance front to rear.
By using the Jeep CJ-7 master cylinder with its slightly smaller bore, you've booster the brake pressure to the rear calipers when compared to a stock Ford Explorer. This is all relative to the brake pedal and booster force. The best test is the real world. Overall, you do not want the rear wheels to lock up under hard braking or on a slick surface. This makes the proportioning valve important.
If you had installed an aftermarket manual proportioning valve on the rear brake system, and adjusted the valve to prevent wheel lockup, the setting would likely be the same as how your brakes apply now. The Jeep CJ-7 chassis dynamics are different than the Explorer, and the wheelbase is shorter. You do not have symptoms of over-braking at the rear wheels...That's how the brakes and the proportioning valve should work!
As a footnote, you shared that you now have "new" rear calipers after the bleeder valve issue. The photo at your earlier post shows an older caliper and rotor. If these parts are used, the brake test results you get now may be different if you install new rotors, pads and calipers. There could be an improvement in rear brake performance with all new parts. If so, be cautious until you're certain that the rear brakes still will not lock up on slick surfaces or under hard braking.