Disc brake conversions are popular, and I cover that topic in my Jeep CJ Rebuilder's Manuals (1946-71 and 1972-86 Editions, Bentley Publishers). Whether the CJ has a four-drum system or a disc front/drum rear system, the master cylinder must be considered during a disc brake conversion.
There are two master cylinder concerns when converting to disc brakes: 1) the piston bore size and fluid volume per stroke of the pedal and 2) any "residual valves" that might have been used for the drum brakes. For disc brakes to work, the master cylinder must have enough fluid displacement to apply the calipers and pads. Disc calipers use more brake fluid per pedal stroke than properly adjusted drum brakes.
If the Jeep is a vintage CJ 4x4 with a single master cylinder and drum brakes, especially the 9-inch diameter drum system, the stock master cylinder will be inadequate for modern disc brake calipers. Drum or disc brakes, I'd want to get rid of the single master cylinder for safety sake, regardless!
In converting to disc brakes, the best choice here should be a modern four-wheel disc brake type dual master cylinder retrofit. A retrofit can even be done using the original, through-the-floor brake pedal, as I illustrate in the 1946-71 Jeep CJ Rebuilder's Manual (Bentley Publishers). I fabricated a safe, sturdy mount for a later dual master cylinder—mounted beneath the floorboard like the stock master cylinder and actuated by the stock brake pedal.
Sometimes, a disc/drum master cylinder will have adequate fluid displacement on the rear drum circuit to operate retrofit disc rear brakes. Again, this depends on the master cylinder's bore size and stroke per pedal application. The rear fluid reservoir is often smaller, so keep fluid at the recommended full level.
On 1972-up Jeep CJs with four-wheel drum or disc front/drum rear brakes, you may be able to use the stock master cylinder with a disc brake conversion. Be aware, though, that some master cylinders will require removal of the residual valve(s) from the master cylinder ports.
The "residual valve" is important on many drum brake systems. To keep the wheel cylinder cup lips expanded, which prevents fluid seepage from the wheel cylinder with the brakes released, a valve is built into the hydraulic system to hold "residual pressure" in the wheel cylinders when the brakes are released. Early single master cylinders and many four-wheel drum or disc/drum dual master cylinders have built-in "check" or "residual" valves.
This residual pressure is below the tension of the brake shoe return springs. Residual pressure is simply to keep the wheel cylinders from drawing air or leaking fluid when the brakes are released. This pressure is typically around 12 PSI, well below brake shoe return spring tension.
By design, disc brake calipers do not require residual pressure. The pads release pressure with the pedal release. There is adequate fluid available in the circuit to apply the brakes without lag or hesitation.
Some disc brake hydraulic systems, do have very slight residual pressure to keep the pads close to the rotors at all times and improve brake response time during pedal application. This pressure would be around 2 PSI and not enough to cause premature pad wear, fade or overheated rotors.
Note: If you're using a four-wheel drum or disc/drum CJ master cylinder, check the fluid line ports for a residual valve. Typically, this valve is simply a rubber plunger and balance spring at the back side of the tubing flare nut seat. With the brake lines removed from the master cylinder, you can see the rubber plunger through the passageway at the center of the tubing flare nut seat. This seat is removable for service and seat replacement. If you are curious how to safely remove the seat, I'd be happy to detail—ask here at the forum!
Caution: When retrofitting from drum to disc brakes, you need to remove the drum brake residual valve(s). Earlier Jeep dual master cylinders for four-wheel drum brakes have residual valves at both the front and rear fluid line ports. OEM disc/drum brake systems can have a residual valve on the rear brake circuit. If the residual valve for drum brakes is left in place, the disc brake pads will drag on the rotors with the brake pedal released. This can cause excessive pad wear, brake fade and even wheel lockup.
One disc brake conversion example is our fellow forum member "LastCJ7". He has a 1986 CJ-7 Jeep (disc front/drum rear factory brakes) and is converting to rear disc brakes. He's trying the CJ-7 dual master cylinder before considering a late Jeep TJ Wrangler Rubicon (four-wheel disc from the factory) master cylinder...LastCJ7 needs to make sure there is no residual valve holding pressure in the rear brake system with the brake pedal released.
On later disc/drum master cylinders, there may not be a residual valve in the rear brake circuit. Many manufacturers have changed over to stiffer wheel cylinder cup expander springs with sturdier cup expanders. This measure keeps the rubber cups expanded with the brakes released and serves the same purpose as older residual valve systems.
When converting to disc brakes, explore whether your original dual master cylinder uses a residual valve or valves. Vintage, single master cylinders have a check valve within the master cylinder to hold residual pressure in the system—one more reason why a single master cylinder is not a candidate for a disc brake conversion!
Make sure the master cylinder's fluid displacement (per pedal stroke) will meet disc brake caliper requirements. If in doubt, retrofit a combination valve and master cylinder from a similar chassis—like retrofitting a Jeep TJ Wrangler Rubicon master cylinder and combination valve to a CJ-7 chassis.
Summing up, make sure the brake hydraulic system is compatible with the disc brake calipers and rotors. Both the CJ-7 and TJ Rubicon are on a 94" wheelbase, each has beam axles and an inline six-cylinder engine, their curb weight is a close match, so they should have similar braking needs and characteristics...Jeep TJ Wrangler Rubicon brake components would be a good template for the CJ-7 wheelbase and four-wheel disc brakes.