How To: Dual circuit brakes early A

[AW-100]

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 in stead, as it has a bit longer pedal travel, lighter pedal force required and a better feel for the brakes, but anyhow.

Througout bench bleeding required, to make sure I got out all the air trapped in the MC.

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 throughoutly, 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

 

[67Dart273]

Thanks for the great photos and write up.

The only issue I have is with your description of fittings. You should specify whether you are talking about the common "call out" size of the fitting, or the THREAD size of the fitting.

For example you say above

"And as you can see, I used a 3/8" joint to join the old pipes with a new one, that I pulled up to the FRONT port of the MC."

That is NOT a "3/8 joint". It is called out as a "3/16 inverted flare union". Some adapters for master cylinders, etc, are pretty much proprietary threads, that is, they were not "proper" inverted flare standard thread size, in which case the thread size AND TPI (pitch) should be specified.

To put this another way STANDARD fittings for inverted flare (and other tube) are always called out by the tube size, as in "3/16 inverted flare nut." That is to say, "the standard flare nut which fits onto a 3/16" tube and therefore has a 3/8" X 24 thread"

Here's some "dope" on fittings

stanleymotorcarriage.com/Parts/images/ParkerFittings.pdf

 

[360DartGT66]

Great!

So if I'm getting this right, the rear port powers the front brakes and the front port powers the rear brakes?

Is there any difference between the ports other than the port thread size?

 

[68gtxman]

Thanks for the step-by-step instructions with photos and parts list! I wonder of the original system had a warning light system (I never had a single type brake hydraulic system)? If so, then switching to a front/rear distribution block will enable sending the signal to the warning light should one side fail.

 

[TrailBeast]

Great!

So if I'm getting this right, the rear port powers the front brakes and the front port powers the rear brakes?

Is there any difference between the ports other than the port thread size?

Disc brakes require more fluid availability than rears as they wear because the pistons in the disc calipers are so much larger than the pistons for the shoe type brake.

Why they designed it so the front reservoir feeds the rear and so on, I believe they did it that way to make the front brakes start applying before the rear. (If I remember that correctly)

 

[360DartGT66]

Ok, I got it. Thanks!

 

[TrailBeast]

Ok, I got it. Thanks!

Sorry, I meant to say they wanted the rears to come on first and I said front.

 

[67Dart273]

I wonder of the original system had a warning light system (I never had a single type brake hydraulic system)? If so, then switching to a front/rear distribution block will enable sending the signal to the warning light should one side fail.

Single systems had no warning light. The lamp indicates a pressure difference between the front / rear, and a "single" system had no such separation

If you wanted one, you'd have to find a later pressure switch / distro block and replace the original.

Frankly I don't see much need. Your foot will tell you there's a problem

 

[AW-100]

Yes, I see what you mean 67dart273.
I've used 3/16" pipes, inverted flare, with the threads 3/8"x24NF.
The junction block has threads 7/16"x24NS.
The MC has threads 9/16"x20 and 1/2"x20.
I'll edit in the numbers.

This old bird does not have any warning light fitted, and neither does the new MC.