Brake bias, caliper BORE size and PAD recomendations...

This is the last manual master cylinder that I used. I got it from Dr Diff.
Nice looking piece... 15/16", Aluminum with plastic reserviors, very light. The fitting sizes were sorta small though.

The 3/8" fittings posed a small problem. The combo block used a larger inlet fitting for the front line. I had to use an adapter.

This is the inlet of the adapter. I'm wondering if this thing caused some sort of restriction...?

Outlet

As stated, the facts elsewhere just dont add up to explain the problems with the manual setup. Pedal ratio would have been the same with the 4 wheel drums that the car had long before.
The front brake line needed to be 3/8" at the master and 9/16" at the combo valve. I am terrible at flaring such small tubing, so I paid a guy to put a 9/16" fitting into a new brake like that had 3/8" fitting on the other end. HIS flaring skills were not any better than mine and the line leaked. THAT is why I used that adapter.
Feets, 383Cuda.... ANYONE have an opinion on the chance that THIS is the problem?
I just can't seem to let this one go.....!

Many master cylinders use larger fittings on one line. However, the lines themselves are the same size. It keeps people from installing the lines incorrectly.

If the adapter you used did not reduce the fluid passage below the size of the line I don't see where it would have been a problem.

I really wish that I had an answer for this. Its been almost 2 years since I started this thread and I still think about it.
Some thought my pedal ratio was wrong for manual brakes to work.
I responded, saying the pedal ratio is identical to what the car had when I bought it with 4 wheel drums.
I tried 4 different non power master cylinders with different bore sizes and every one sucked. Hard pedal and poor stopping.
I thought of taking a spare brake pedal and drilling a new hole, higher toward the pivot point. This would increase the ratio. This would make for less effort, but would it be enough?

In part it comes down to weight and size. Your B body is heavier than the A you are comparing to so that gives the A an advantage.

On line pressure, making the master bore smaller increases pressure but risks running out of volume. In the spirit of being a bit conservative I wouldn't want to go smaller than recommended and maybe sometime run out of volume. Also I wouldn't personally modify the linkage. Lots of design and testing went into it to make it robust.

Also pads. For around town driving with cool brakes, the low cost organic pads have more bite than anything. The higher end DD type pads also have good bite (think parts store "gold").

Standard grade semi-metallic and performance pads have the least bite when cool... but hang in there well when hot.

Also vacuum. Sounds like you have a bigger cam and lower vacuum. PB work best with 18+ vacuum. Anything over 10 or 12 is usually enough for putting around but max assist needs more vacuum. One test would be to get up to speed and downshift so the engine is spinning>2000 RPM with throttle closed. At that point it should be making good vacuum. Are the brakes noticeably better?

In your case, I'll suggest maybe a pad change would help or more boost. An 8" double booster would give roughly 50% more boost compared to a 9" single. Or hydroboost which would also take vacuum out of the equation.

I've got a similar opportunity. I've got 13" Baer brakes up front. Piston area is smaller than OE and the 9" booster does not have enough "oomph". The brakes work very well but I run out of boost before I run out of brakes. I'll likely do the double booster or hydroboost. My preference is the hydroboost as its lighter, more compact and provides more pressure. It's just spendy.

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.

It's basic math and physics.

I will run thru it for you.

For a master cylinder you apply a force to the piston which tries to compress the brake fluid. The pressure increase in the brake fluid is determined by the surface area of the piston and the force you apply to it.

The formula for determining the area of the piston is 3.1415 time the radius squared.

For a 1 inch bore master cylinder the area is 0.78 square inches.

For a 15/16 inch bore master cylinder the area is 0.69 square inches.

How does the smaller bore makes more pressure? You are applying the same force to both. To start with let us assume there is a third bore size that equals 1 square inch of area. Apply 100 lbs of force to that you get 100lbs per square inch. Since the two examples are smaller than 1 square inch you have to calculate how much pressure is being created by understanding that the force applied to each example needs to be corrected to account for the smaller bore size in relationship to one square inch. The formula for that is to divide 1 by the area of the bore you are using.

1 in bore is 1/0.78 = 1.27

15/16 inch bore is 1/0.69 = 1.44

These are the ratios to correct the smaller bore sizes so that they would match a 1 square inch area. We do this so that the end value (pressure out of the master cylinder) can be read as pounds per square inch.

Now let's apply a force to these two sizes, we will use 100 lbs of force, your foot, to make the math easy.

The formula at this point is 100 (force from your foot) times the ratios we calculated.

1 in bore is 100 * 1.27 = 127 lbs per square inch.

15/16 inch bore is 100 * 1.44 = 144 lbs per square inch.

Now the caliper end of things works similarly, but in the opposite direction.

You have a 2.75" caliper piston bore we will need to calculate the area, same formula as above for area, 3.1415 time the radius squared.

Radius of 2.75 is 1.375, square it then times 3.1514 equals 5.94 square inches of piston area.

If you apply the 127 PSI from the 1" bore master cylinder to the caliper piston you get 754 psi on the caliper piston. (formula is pressure into the caliper times caliper piston area)

If you apply the 144 PSI from the 15/16" bore master cylinder to the caliper piston you get 855 psi on the caliper piston.

It's all a case of hydraulic leverage. We will not be getting into the mechanical leverage imparted by the brake pedal pivot points.

An addendum to the above regarding multiple piston calipers. You have to calculate the area of all the pistons involved (some setups use staggered piston sizing) then add those together to come up with the total caliper piston area then you can do the pressure times area math.

For example, some Mustang calipers use two 1.5" pistons. In this case it would be ((3.1415 times the radius squared) times two) so 1.76" area for each piston times two pistons equals 3.54 square inches total. This is much smaller than the 5.94 square inches of the Mopar 2.75" bore caliper.

So pressure on the pistons will be less. But that application (96 Stang) uses a 13" rotor so force at the wheel may be comparable, not doing that math, lol.

Thanks, guys. I'm just very curious and the boundaries of a non face-to-face meeting make it hard to exchange ideas sometimes.
I do understand the math of it. I've read the contributions Feets provided as well. I'm getting the impression that the leverage of the larger sized rotors is where the majority of the improvements are. The thicker rotor width provides a longer duration before they overheat. Newer cars have greater development of pad compounds available too from what I gather.
Since this is a classic car forum, I figure that most of us are modifying old cars one end at a time. Years back, I pulled the stock and perfectly functioning 10" rear drums to install the Dr Diff 10.7" rear disc kit. This was done purely for appearance reasons. I wanted the look of a disc rotor through the wheel spokes. It turns out it didn't make any difference in how the car stopped. Braking performance was the same. That wasn't a bad thing....the brakes weren't terrible before or after. I went from what was probably a 15/16" wheel cylinder to a 1.5" single piston caliper. This is surely apples to oranges but I recall reading that when one had rear wheel lockup with drum brakes, the trick is to use smaller wheel cylinders. Well, a larger sized caliper piston would seem to be expected to provide more clamping force there too.

If you were closer I'd just have you drive my Coronet so you could see what you think. I can tell you that my Coronet stops better than my Toyota 4Runner which has factory power 4 wheel disc brakes. I can exit the freeway at 80 mph in the Coronet and it will immediately pull down to 35 or 25 as I approach stopped traffic. With the 4Runner I have to be careful to leave plenty of stopping distance when slowing down from anything over 70 mph.

My Coronet has 13 inch front rotors with Porsche calipers. I added a rear disc kit a few years back but I think the factory drum brakes that I had on there before worked better than the disc brakes. I had the big station wagon drums out back and they really worked.

That is what I want....brake confidence like I have felt in newer cars. The 72 Duster stops better than any classic car I've owned and it is a basic manual MC 11" disc 10" drum setup with 2.75" calipers. That car is what gives me hope that I can make the Charger stop as well or better if the right selection of parts is found.
Dr Diff suggests his 13" Cobra front kit and the 11.7" rear along with either a 15/16" manual MC or a hydroboost with a 1 1/8" MC.

If you can lock your brakes up you already have more brakes than tire.

You can update your brakes all day long, but unless you have the tires to use that added braking power you are wasting time. So unless you need the added thermal mass of larger brakes look to tire size increases first.

If you already have fitting the biggest tire you can then yes it makes sense to update the brakes, assuming you can no longer lock them up, lol.

Tire size is not a problem.

I don't recall the last time that I was able to or actually did skid the brakes. Right now the engine is out for a rebuild but before I tore it down, the brakes worked okay but weren't impressive enough to brag about.

I think the newer cars can use smaller calipers because of the larger rotors.
Can't underestimate the leverage adder, plus as mentioned, I'm sure the new pads are much better than the old stuff.

Just think of how easy it is to break a bolt free after you slide a pipe on the rachet to extend it.....

and speaking of pads, how do the ceramic types compare to the semi-metallic of back in the day, as compared to "red stuff", "yellow stuff", etc that EBC offers ?
this is in relation to the small rotor size needed to fit 15" wheels.
i have read all the descriptive attributes of the above mentioned pads, as well as what is offered by Hawk and other race type pad manufacturers, but i am hesitant to pull the plug on a set of pads, only to find out they may or may not, work to my expectations.

Excellent point. Could it be that simple?

ABS probably has an effect as well. While I can no longer remember the details I do recall an article Eberg wrote comparing the brakes on his Green Brick vs sports cars of the time it was written. Can't seem to google it though. I don;t recall it being real fancy though it might have has one of AndyF's Viper caliper setups on it.

How much does that Duster weigh?
How much does your Charger weigh?
Bigger cars need more brakes(bigger contact area) to stop as good as lighter smaller cars do, basic physics

This pad part of the thread seems to be getting lost and may need a thread of its own...

good point.

The brake pad topic is a valid one and supports the inclination to change to a modern system. The Dr Diff stuff is based on OEM production calipers and rotors. This likely means that the brake pads might be available from the higher end aftermarket suppliers.