Sounds like adding an electric vacuum pump would fix it for good. 18"+ all the time. Master Power Brakes, Summit or others carry them. A few cars had them as original equipment so it may be possible to find one in a JY.

I don't understand why you had a problem with manual brakes.

I've got large diameter multi-piston manual brakes and the pedal feel is equivalent to that of the modern performance cars I deal with at work.

maybe i missed it, but you do or do not have a rear proportioning valve? if im thinking about this correctly, rear disk would want much higher line pressure than rear drums.

I agree.
The system is plumbed like a 4 wheel drum setup with what was originally a drum distribution block. I tried 4 wheel drum versions, a few disc/drum versions and found zero difference between any of them.
The electric vacuum pump may be the answer. I'd prefer to simplify the system but unless I can figure out the reasonong for the poor performance of the manual setup, it seems that I am limited in my choices.

It's kinda funny. As your master cylinder size goes up your pedal effort goes up but the brake pressure goes down. [/quote]

Here I am, ten years later and I'm revisiting this same issue.
I have the engine, trans, K member and suspension out of the car. I pulled the power brake booster and MC and swapped in another 15/16" MC from Dr Diff but I have a drum-drum distribution block.
A video online featuring a crash involving a 64 Mercury Comet really shed light on the importance of upgraded brakes for high performance cars.
I'm also looking into the Dr Diff 13" front brake kit.

I've been using 13 inch front rotors on my Mopar projects for more than 20 years. Maybe close to 30 years, I've actually lost track on when I did my first big brake conversion. The 13 inch front rotors make the car so much easier to drive that I doubt I'd ever go back to a factory size brake. Especially when it is so easy to make power these days. The engine in my Coronet made over 600 hp even though it has a very modest hyd roller cam and the compression ratio is 9.8 for pump gas use. With that much power the car needs good brakes. It is just too easy to get into trouble with 600 hp on tap.

Andy, maybe you can help me understand something.
The Dr Diff Cobra kit uses a 2 piston caliper with 1.5" bore calipers. The math states that the surface area of the pistons is 3.53 square inches. My 2.75" single piston iron caliper has a surface area of 5.93"
I'm having trouble understanding how the 2 piston caliper with a smaller surface area can provide more clamping force than the big single piston caliper.
I thought about it and was thinking that maybe the 2 piston calipers require less fluid volume to move the same amount that the larger one does.

Nothing wrong with your math, the factory 2.75 piston is huge compared to anything used in later model cars or the aftermarket. That is why you have to go with a smaller piston in the MC when you switch to aftermarket brakes. Everything has to be smaller to work out correctly. Use a smaller MC piston and use a larger rotor (I prefer 13 inch rotors with 17 inch rims) and you'll have a much better braking system than anything the factory used during the muscle car era.

Thank you.
I have 18" wheels so the 13" front rotors and caliper will clear easily. Finding a spare that will clear may be tricky though!

I am starting to understand it a little better but it still seems strange that a smaller surface area in terms of piston caliper sizing can provide more clamping force.

haven't read the whole thread,
I would think the larger rotor would yield the biggest improvement, even with a small caliper
Leverage is king, the further outward the caliper, the better.

"Give me a long enough lever, and I can move the world"

Not sure said that, but you get the idea

It doesn't, in the caliper.

Andy said in the Master cylinder though.

Here is where I am hung up..
My car currently has the Cordoba 11.75 rotors and 2,75" single piston front calipers. Going to a larger rotor makes sense regarding leverage. I am trying to understand how a caliper with less surface area could provide equal to or greater clamping force IF the master cylinder size is the same for both.

Not sure where you got the idea a smaller bore caliper will have more clamping force.

It won't. The smaller pistons will reduce clamping force. You need to reduce the MC bore size to rebalance the system and then you need to change the rear wheel cylinder size so it isn't isn't out of whack. Most people fail at either step one or step two of this process. You at least are thinking about it so you're a step ahead of most.

I am trying to understand how and why a switch from a 12 inch setup to the 13 inch setup would provide a gain.
If the larger rotor provides more leverage but the smaller caliper size isn't as effective, why wouldn't they increase the caliper size ? Is there so much gain in the leverage that it greatly offsets the loss from a smaller caliper?

the brake info sticky at the top by member feets is a very good source of understanding the workings of the brake system.
at least in my opinion it is.

You are mixing and matching parts so there is no logic to the question. If you look at a late model car with 13 inch rotors you'll see that the MC bore size and the piston caliper sizes are all on the small size. The engineers figured out a few years back that if everything was smaller then it could all be lighter but still provide the clamping forces required to stop the car. The old 2.75 caliper used by Mopar back in the 70's doesn't really exist anymore. Now everything uses multiple pistons and small bore sizes.

We must be speaking different languages here because no matter what I write, I'm not getting the answers directly to what I am asking. I feel like I'm dealing with lawyers here.
I have mixed and matched stuff in the past but am trying to get a matched system going BUT want to know why and how it works. Don't you guys have any natural curiosity about how things work? Maybe you don't understand it either but won't admit it?
Andy...I read your response about smaller and matched components on newer factory systems. I'm trying to understand why the old setup could have had a boosted 15/16" master cylinder and 2,75" calipers and an aftermarket setup can have the same sized MC, smaller calipers (Larger rotors) and somehow provide better stopping. Even the stock manual disc/drum A body cars had a MC with a 1 inch bore or something close to that.
I'm not broke and pinching pennies here.....I just like to know that I'm not spending time and money to see no results. I want to know the how and why. I'm like this with everything even if it is free.

The new designs are more rigid so you end up with better brakes even though the clamping force might be a little less. The old school mopar cast iron 2.75 inch caliper isn't super rigid so there might be some clamping force lost just due to the design. The cars that I've built over the last 10 years have all had 13 inch rotors (1.25 thick) with either Viper or Porsche calipers. I then use a small 15/16 diameter MC and if I'm using drum brakes in the rear I use the smallest wheel cylinder out back. Those systems work great. The brake pedal feels very firm and the car sucks down hard when you lean on the brakes. The factory 11.75 systems that I've used in the past didn't stop nearly as well. Some of the difference might also be due to brake pad material. There aren't many options for the old mopar calipers while you can find at least a dozen different pad compounds for a Viper caliper. Porsche big red calipers have even more choices.