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Re: Brake bias and caliper BORE size.
[Re: Kern Dog]
#1233091
05/16/12 02:28 AM
05/16/12 02:28 AM
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Joined: Jan 2003
Posts: 28,067
Irving, TX
feets
Senior Management
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Senior Management
Joined: Jan 2003
Posts: 28,067
Irving, TX
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If you're running a power pedal with a manual master you're losing pressure.
Either find a manual brake pedal or move the brake push rod higher on the pedal.
Measure from the center of the brake pedal pivot to the middle of the brake pedal pad.
Measure from the center of the brake pedal pivot to the center of the pin where master cylinder push rod connects to the pedal.
It's needs to be a 5:1 ratio for factory sized master cylinders to work.
If you have a power pedal the leverage is greatly reduced. A "correct" sized master cylinder will require a lot more leg effort to get the same brake pressure.
To calculate line pressure do the math.
(input force * pedal ratio) / surface area of master cylinder piston
Here's a list of master cylinder sizes and line pressures using a 4:1 pedal ratio and a 150 lbs pedal force:
3/4" = 1364 psi
7/8" = 1000 psi
15/16" = 869 psi
1" = 759 psi
1-1/8" = 606 psi
1-1/4" = 488 psi
Now, using a 5:1 pedal ratio and 150 lbs pedal force you get:
3/4" = 1705 psi
7/8" = 1250 psi
15/16" = 1086 psi
1" = 949 psi
1-1/8" = 758 psi
1-1/4" = 610 psi
Does that help?
Your 15/16" master cylinder with a 4:1 ratio and pedal effort of 172 lbs gives you 1000 psi.
Assume you weigh 172 lbs. Stand on one leg. Do a deep knee bend. The amount of force needed to stand up straight again with one leg is how hard you have to push on the pedal to get the pressure listed above.
You want a minimum of 1000 psi working pressure.
To get 1200 psi for a comfortable margin you have to push with 207 lbs of force.
Either fix the pedal or find a 3/4" master cylinder and hope it has enough travel to make up for the pistons moving.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: Brake bias and caliper BORE size.
[Re: feets]
#1233092
05/16/12 04:08 AM
05/16/12 04:08 AM
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Joined: Feb 2010
Posts: 18,493
Granite Bay CA
Kern Dog
OP
Striving for excellence
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OP
Striving for excellence
Joined: Feb 2010
Posts: 18,493
Granite Bay CA
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Feets, I'm impressed and honored. That post took some effort and I thank you.
This car started out with manual 4 wheel drums. I converted it to Power front disc/rear drum, then to Power 4 wheel disc. Now it is a non power 4 wheel disc. It has the same brake pedal that was in the car when I bought it. I'll have to check and calc the pedal ratio, then report back.
I seem to remember that some cars had 2 holes in the brake pedal lever, the top one was supposed to be for manual brakes. I don't recall seeing any 67-76 A bodies that were like this and my Charger certainly isnt.
On another note I had the idea of eliminating the combination block, thinking that there may be a better way. I was thinking of running the M/C rearmost port to a TEE which will split off to each front wheel, then run the forewardmost M/C line to the rear with an adjustable proportioning valve in line. My thinking is that THIS may be a "work around" to a possibly defective combo valve. I posted this question elsewhere and a few suggested simply gutting the combo valve and keeping it. I took a drum/drum and a disc/drum combo valve and took the parts out. I blew compressed air through both and found that even with the guts removed, the front and rear ports remained seperate!
The combo valve mods are moot if my pedal ratio is waaay off. Another thought: Since the car WAS 4 wheel drum, and since drums require LESS volume to operate, maybe the existing pedal ratio is simply not compatible with a 4 disc system.
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Re: Brake bias and caliper BORE size.
[Re: feets]
#1233093
05/16/12 04:35 AM
05/16/12 04:35 AM
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Joined: Feb 2010
Posts: 18,493
Granite Bay CA
Kern Dog
OP
Striving for excellence
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OP
Striving for excellence
Joined: Feb 2010
Posts: 18,493
Granite Bay CA
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Quote:
5:1 pedal ratio for manual brake cars.
4:1 pedal ratio for power brake cars.
I think that's right. If not, the info was worth what you paid for it. 
Okay, I'm back. Using the instructions given, my pedal ratio is quite different.
I measured from the center of the brake pedal pad to the center of the point at which it swings...12 3/8" The measurement from the pivot to the center of the brake pushrod bolt is 1 7/8". This works out to a 6.6 ratio. I had a mid 70s A body pedal assembly nearby and looked at it for comparison. I got 11 1/4" and 1 5/8" for a ratio of 6.92. I measured AND did the math twice. For the Charger, the brake pushrod would've needed to be 3 1/32" away to be at a 4 to 1 ratio or 2 1/2" away to be close to the 5 to 1 number.
It was worth a try.
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Re: Brake bias and caliper BORE size.
[Re: runinonmt]
#1233098
05/16/12 02:28 PM
05/16/12 02:28 PM
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Joined: Feb 2011
Posts: 4,302
Nebraska
72Swinger
master
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master
Joined: Feb 2011
Posts: 4,302
Nebraska
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I have 74 Dart front calipers,Dr Diff 15/16" MC,11" rear drums and a factory original to my car Drum/drum prop valve and my car stops as good as anything i've ever driven with no rear lockup. My pedal and rod are also orignal manual stuff. If you want I can measure my pedal ratio. It sounds to me like my setup is identical to yours other than the rear discs.
Mopar to the bone!!!
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Re: Brake bias and caliper BORE size.
[Re: DoctorDiff]
#1233099
05/16/12 02:52 PM
05/16/12 02:52 PM
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Joined: Feb 2010
Posts: 18,493
Granite Bay CA
Kern Dog
OP
Striving for excellence
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OP
Striving for excellence
Joined: Feb 2010
Posts: 18,493
Granite Bay CA
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Quote:
Only pre '66 B-bodies used a different pedal for power brakes.
I have a few 7/8" bore master cylinders, but that seems kind of extreme.
You probably don't need any rear proportioning with your particular disc/disc combination. Did you try a simple drum brake distribution valve?
Can you get more aggressive front brake pads?
I have tried 2 different combination valves, a 75 Dart disc/drum and a drum/drum. The drum/drum valve is in place now. Both delivered the same feel.
I may have not mentioned it, but it sorta feels as if the front end isn't getting enough line pressure. On other factory based master cylinders, the outlet fittings are different sizes to ensure the proper front/rear placement. THIS 15/16" master has similar ports front and rear. I have the rearmost reservior connected to the front brakes like a factory application.
MORE aggressive front pads? That is an idea. I also have been asked if the current new pads have been bedded in yet. Simply put, if the braking im proved by 20%, they would still suck. It really is that bad. What are the chances of having TWO combo valves with internal blockages? I have considered plumbing the lines without a factory valve. 
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Re: Brake bias and caliper BORE size.
[Re: Supercuda]
#1233101
05/16/12 08:33 PM
05/16/12 08:33 PM
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Joined: Feb 2010
Posts: 18,493
Granite Bay CA
Kern Dog
OP
Striving for excellence
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OP
Striving for excellence
Joined: Feb 2010
Posts: 18,493
Granite Bay CA
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Quote:
The power brake m/c for a 75 Dart looks to be 15/16".
the manual one is 1 1/32".
Either your new m/c is flaky or you need to upsize it.
I owe you an apology!
I just removed the M/C from the power booster that I previously had on the car. The bore WAS 15/16". To confirm, I pulled the M/C from a 73 Dart power disc car. It too was 15/16". I checked my NON power A body factory disc master and it looked like 1 1/16" but it could have been 1 1/32"... hard to tell when they are that close.
This is odd. Maybe the increased pedal ratio of the manual setup overcomes the bigger bore?
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Re: Brake bias and caliper BORE size.
[Re: Kern Dog]
#1233102
05/16/12 11:05 PM
05/16/12 11:05 PM
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Joined: Sep 2007
Posts: 14,889
up yours
Supercuda
About to go away
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About to go away
Joined: Sep 2007
Posts: 14,889
up yours
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Well, I am just a piece of carp anyway, lol.
I am running manual brakes on my 65 Cuda, 11.75" disc/11" drums. Running the M/C out of a late M body, forget the diameter offhand, but the pedal is not as firm as I would like. I think it's a bleeder on the caliper location issue but haven't gotten around to verifying it yet.
I forget, it's been years, exactly what caliper I am running, I think M body and I flipped the calipers to clear the swaybar. This might have put the bleeders in less than optimal position causing my issue. Are you 100% sure the bleeders are at the very top of the caliper bore?
They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.
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Re: Brake bias and caliper BORE size.
[Re: Supercuda]
#1233103
05/17/12 12:34 AM
05/17/12 12:34 AM
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Joined: Jan 2003
Posts: 28,067
Irving, TX
feets
Senior Management
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Senior Management
Joined: Jan 2003
Posts: 28,067
Irving, TX
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The hot rod has 8 piston calipers up front. The total working area is 4.40 square inches. The rear 4 piston calipers have a working area of 2.05 square inches.
I use a 1-1/8" master cylinder on the stock manual brake pedal. I don't remember the pedal ratio.
This setup have the best feeling manual brakes I've ever driven. The pedal isn't hard to push and the car slows down as soon as the pedal moves. It's a beautiful thing. Despite soft 245/45-18 and 295/45-18 tires I can lock the brakes without excessive pedal pressure.
My situation is a little different with 14.25" front rotors and 13" rear rotors.
Your front calipers have a piston area of 5.94 square inches and the rears are 2.43 square inches.
Both calipers displace more fluid than mine. Your rotors are 11.75" front and 10.7" rear.
I would have to sit down and calculate the difference in brake force between my setup and yours assuming we have the same pedal ratio.
I might have to step out into the shop and measure my pedal.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: Brake bias and caliper BORE size.
[Re: feets]
#1233104
05/17/12 01:10 AM
05/17/12 01:10 AM
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Joined: Feb 2010
Posts: 18,493
Granite Bay CA
Kern Dog
OP
Striving for excellence
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OP
Striving for excellence
Joined: Feb 2010
Posts: 18,493
Granite Bay CA
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Quote:
8 piston calipers up front. The total working area is 4.40 square inches. The rear working area is 2.05 square inches.
This makes the rear caliper force about 46 % of the front.
Your front calipers have a piston area of 5.94 square inches and the rears are 2.43 square inches.
Both calipers displace more fluid than mine. Your rotors are 11.75" front and 10.7" rear.
Using the same math, my numbers show the rear calipers here have 41 % of the front
I guess this means that I would have a little less rear bias, which I call a GOOD thing!
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Re: Brake bias and caliper BORE size.
[Re: feets]
#1233105
05/17/12 10:06 AM
05/17/12 10:06 AM
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Joined: Oct 2003
Posts: 21,345
Marysville, O-H-I-O
70Cuda383
Too Many Posts
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Too Many Posts
Joined: Oct 2003
Posts: 21,345
Marysville, O-H-I-O
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Quote:
The hot rod has 8 piston calipers up front. The total working area is 4.40 square inches. The rear 4 piston calipers have a working area of 2.05 square inches.
I use a 1-1/8" master cylinder on the stock manual brake pedal. I don't remember the pedal ratio.
This setup have the best feeling manual brakes I've ever driven. The pedal isn't hard to push and the car slows down as soon as the pedal moves. It's a beautiful thing. Despite soft 245/45-18 and 295/45-18 tires I can lock the brakes without excessive pedal pressure.
My situation is a little different with 14.25" front rotors and 13" rear rotors.
That's not bad at all! for comparison, the Gen 2 viper calipers have a combined surface area of 3.88 square inches, and used a 1 1/16" master cylinder with a power booster. I'm running those calipers with a 1 1/8" master on my power brake booster on the Dakota.
I love how they borrow parts from each other too! the 03/04 dakota master cylinder is exactly the same as the gen III Viper master cylinders, and the 97-02 Dakota master, the 92-02 Viper master, 03-12 Viper master, and 03/04 Dakota master, all have the same piston depth into the brake booster, the same bolt pattern to bolt the master to the booster, and thus, all 4 masters are interchangable! (with the exception that the gen 2 viper masters use SAE double flare fittings, and the other 3 use metric bubble flare fittings)
with my brakes on the booster, it feels really good. If I was to manual brakes, I'd probably find a disc/disc master that was 1 1/16" with these viper calipers.
**Photobucket sucks**
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Re: Brake bias and caliper BORE size.
[Re: Kern Dog]
#1233106
05/17/12 12:44 PM
05/17/12 12:44 PM
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Joined: Jan 2003
Posts: 28,067
Irving, TX
feets
Senior Management
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Senior Management
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Posts: 28,067
Irving, TX
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I crashed some numbers around last night but lost my internet connection and couldn't post 'em. I compared the hot rod to your car. This wasn't an attempt to make my car look good. The hot rod's brakes work well and I could find all the numbers involved. The pedal ratio on the hot rod is 7.125:1 so my guess was WAY off. I looked at the pedal ratio, MC bore size, and effective brake radius of the cars. With the hot rod, 100 lb pedal effort makes 4883 lb/ft of front brake torque. With your car 100 lb pedal effort makes 4961 lb/ft of front brake torque. The 1.5% will never be seen in the real world. Comparing master cylinder volume to brake caliper volume will show some interesting numbers. When my master moves 1 inch the front calipers move out .1129". When your master cylinder moves 1 inch the calipers move .058". The resulting pedal feel will be quite different. Brake pads are usually a few thousandths of an inch off the rotors unless the rotor is not round. If you have pad material built up on the rotor or lots of runout the pads will get knocked back away from the rotor. Shifting to the rear.. The hot rod turns 100 lb pedal effort into 2156 lb/ft of brake torque. Your car turns 100 lb pedal effort into 1355 lb/ft of brake torque. The 38% difference will be very noticeable. When my master cylinder moves 1" the rear caliper pistons move .242". When your master cylinder moves 1" the rear caliper pistons move .142". ***EDIT***
I butchered some of those numbers. oopsie. The comparisons between the two systems are the same so you can still learn what you needed to learn. See this post for more accurate info.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Subscribing so I can find this thread again later when I look at my brake issues again.
) in my truck, and they just don't seem to have any 'bite'.