My 1965 Plymouth Barracuda came with single circuit brakes. Here's how I converted the old girl to dual circuit.
Old single circuit MC.
I started out with a Raybestos MC36221 for a 1967 Barracuda, original with dual brakes.
Later research shows that I might have used a MC36338 for the 1971 Barracuda instead, as it has a bit longer pedal travel, lighter pedal force required and a better feel for the brakes, but anyhow.
Thorough bench bleeding required, to make sure I got out all the air trapped in the MC. Just route two pipes into the fluid chambers, fill'em up, and push in the piston several times until you see no more air coming through the pipes.
The Barracuda is originally fitted with 3/16" inverted flare tubing, with 3/8"-24NS threads. From the brake junction box below the MC, I pulled the tube traveling to the rear of the car, and plugged the junction with a 3/16" inverted flare plug fitting with 3/8"-24NS threads.
And as you can see, I used a 3/16" inverted flare joint to join the old pipes with a new one, that I pulled up to the FRONT port of the MC.
Now the original MC was fitted with a 1/4" tube, with 7/16"-24NS threads. I bought 3/16" tubes, so I needed a 7/16"-24NS to 3/8"-24NS adapter to the top port of the junction box.
This pipe was pulled up to the REAR port of the MC.
Unfortunately, I had bought a bit short tubes, only 20" long, so I was unable to add a spiral to the tubing to take up vibrations. But I think I got so much slack in the tubing that it'll be OK.
Now the MC had a front port with 9/16"-20 threads, and a rear port with 1/2"-20. Hence I needed adapters, 9/16"-20 to 3/8"-24 and 1/2"-20 to 3/8"-24.
The MC36221 was bolt-on, and fitted directly into the old MC bolt pattern.
Now that I had all of the brake lines already drained for brake fluid, I also swapped all three of the brake hoses with new ones, and new hose clips.
Refilled the system, bled it thoroughly, and there you have it.
I used these parts:
Master cylinder Raybestos MC36221
2x 3/16" inverted flare brake lines, I bought 20", but recommend longer ones.
3/16" inverted flare plug fitting for the junction
3/16" inverted flare joint
7/16"-24 to 3/8"-24 inverted flare adapter
1 / 2"-20 to 3/8"-24 inverted flare adapter
9/16"-20 to 3/8"-24 inverted flare adapter
New brake hoses
New brake hose clips