Dr Diff 13" "Cobra" front disc brakes.....who has experience with them?

Kern Dog

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I'm looking at the Cobra front disc brake setup that Dr Diff offers.
Funny story...Back in 2006 when I bought his 10.7" rear disc kit, he offered to let me use a prototype of this front 13" kit at no expense. He said If I wanted to keep it, we'd work something out. He was looking for real world feedback on whether it was worth further investment for sale to the public. I politely declined because I was sure that I would like it but didn't want to spend the money for it at that time.
Here we are, 15 years later and I am ready now.
I have had the ubiquitous 12" Cordoba rotors (Actually 11.75) on 75 Dart knuckles with single piston iron calipers with a 2.75" bore pushing carbon metallic pads. It stops okay but not fantastic like some stock front disc-rear drum A body cars that I have had.
I want to change to a NON power master cylinder at the same time. I have been using this bulky booster and master cylinder for over 20 years now...
Brake 1.JPG


The Dr Diff master cylinder I want to use looks like this...

Brake 7.JPG

Aluminum, 15/16" bore and 2 bolt mount. It comes with a 4 to 2 bolt adapter plate.

The Cobra kit uses an aluminum hub, a lighter rotor and a twin piston aluminum caliper. The weight savings could be 10 lbs per side compared to what I have now while providing better braking.
Who has experience with this kit?
 

Don67Satellite

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Here's it installed. I really decided on this one after a friend recommended this style, and the one from Dr. Diff was perfect for a manual brake car since the bore has the groove, and comes with the rubber grommet to fit the stock manual brake push rod. No issues at all, plug & play.

IMG_0442.jpg
 

Kern Dog

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In 2012, I wanted to try a manual brake setup for the sake of simplicity and weight savings. I tried 4 different master cylinders with the following bore sizes....1 1/8", 1 1/32" and 2 that were 15/16". One of those was a Dr Diff aluminum unit. NONE of them gave braking performance on par with the power system I had BUT I now know that it was probably because I stupidly had a disc/drum proportioning valve in the car instead of a drum/drum block. 4 wheel disc systems are like 4 wheel drums....the proportioning is determined by the caliper or wheel cylinder sizes. Having a proportioning valve in a 4 wheel disc system makes the rear brakes almost useless.
 

Don67Satellite

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I am in a slightly similar boat, but different, as I put this in to replace the small drum/drum master, as my car has Wilwood's on the front. But it's only part of the puzzle for me, as I still need to get rid of my distribution block and get that one piece proportioning valve he sells as well since I plan on just keeping drums on the back. New lines may be in order at some point too.
 

hunt2elk

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Here's it installed. I really decided on this one after a friend recommended this style, and the one from Dr. Diff was perfect for a manual brake car since the bore has the groove, and comes with the rubber grommet to fit the stock manual brake push rod. No issues at all, plug & play.

View attachment 1295437
I am using the same master cylinder on all 3 of my cars. Curious as to why you had to use the brass adapters? None were needed with mine. I did however have to buy replacement Dorman caps and seals. Cass had warned me that was an issue on these master cylinders.
 

Charlie Brown

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My 64 has the following Dr. Diff brake parts / systems.
Front - stage 2 - 11.75 discs with 2 piston calipers
Rear - 10.7" discs / Cobra calipers
Manual 15 / 16 master cylinder
4 wheel disc brake distribution block
This is a manual (un-boosted) system
Observations - I bought this car with the above parts already installed. The looks are great. However, in my opinion, the car does not stop like it should. Car stops straight. Pedal is firm and high but it takes huge leg effort to slow down quickly. I'd be concerned if I had to do a panic stop.
First thing I had to do was to replace the M/C caps / cap seals - original ones did not seal properly. I checked the pedal ratio and found it to be 6:1. I had another pedal from a parts car which I modified to have a 7:1 ratio to increase line pressure. Not much change. In a conversation with Cass, he recommended that this system may need to be boosted either by a brake booster or by a hydro boost system. Since my engine has a "large" cam with not a lot of vacuum, hydro boost seems the best option for me.
If one were to add a brake booster to an un-boosted system, you would have to change your pedal ratio (typically 4:1 ratio) as well as your master cylinder to a larger bore (if using a 15/16 unit).
I'm still wrestling with my current system.
 

Don67Satellite

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I am using the same master cylinder on all 3 of my cars. Curious as to why you had to use the brass adapters? None were needed with mine. I did however have to buy replacement Dorman caps and seals. Cass had warned me that was an issue on these master cylinders.

Really? He didn't mention there were any issues. The brass adapters were just "there", from the old master, and they just threaded right in nicely. Not used for any specific purpose. Things may change when I do a complete line replacement over the winter. Which caps did you replace them with @hunt2elk ?? Also, I'd like to make sure of something. In the above photo posted by @Kern Dog the master has "FILL TO BOTTOM OF RINGS" on the side. Now, what rings are they referring to? To the bottom of the cap filler, or the centre oblong rings that run around the middle and separate the upper/lower part of the tank? Just curious to know if I have mine filled enough or not.
 
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mgoblue9798

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I'm looking at the Cobra front disc brake setup that Dr Diff offers.
Funny story...Back in 2006 when I bought his 10.7" rear disc kit, he offered to let me use a prototype of this front 13" kit at no expense. He said If I wanted to keep it, we'd work something out. He was looking for real world feedback on whether it was worth further investment for sale to the public. I politely declined because I was sure that I would like it but didn't want to spend the money for it at that time.
Here we are, 15 years later and I am ready now.
I have had the ubiquitous 12" Cordoba rotors (Actually 11.75) on 75 Dart knuckles with single piston iron calipers with a 2.75" bore pushing carbon metallic pads. It stops okay but not fantastic like some stock front disc-rear drum A body cars that I have had.
I want to change to a NON power master cylinder at the same time. I have been using this bulky booster and master cylinder for over 20 years now...
View attachment 1295430

The Dr Diff master cylinder I want to use looks like this...

View attachment 1295431
Aluminum, 15/16" bore and 2 bolt mount. It comes with a 4 to 2 bolt adapter plate.

The Cobra kit uses an aluminum hub, a lighter rotor and a twin piston aluminum caliper. The weight savings could be 10 lbs per side compared to what I have now while providing better braking.
Who has experience with this kit?
I think I would talk with Cass, then swap to whatever master he recommends for the current system before spending the money for 1 " bigger rotors. The 11.75 rotors were used on trucks and c bodies as well. I don't think that is where your brake performance issue is.
 

hunt2elk

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Really? He didn't mention there were any issues. The brass adapters were just "there", from the old master, and they just threaded right in nicely. Not used for any specific purpose. Things may change when I do a complete line replacement over the winter. Which caps did you replace them with @hunt2elk ?? Also, I'd like to make sure of something. In the above photo posted by @Kern Dog the master has "FILL TO BOTTOM OF RINGS" on the side. Now, what rings are they referring to? To the bottom of the cap filler, or the centre oblong rings that run around the middle and separate the upper/lower part of the tank? Just curious to know if I have mine filled enough or not.
These are what Cass told me to buy. They are the cheapest at Rock Auto if you can add them to another order. I have a 15/16" aluminum masters on all 3 of my cars, and they all leaked. Put these Dorman caps and seals on, and knock on wood, they have been fine.

20220607_162802.jpg
 
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Kern Dog

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I think I would talk with Cass, then swap to whatever master he recommends for the current system before spending the money for 1 " bigger rotors. The 11.75 rotors were used on trucks and c bodies as well. I don't think that is where your brake performance issue is.
Thank you for responding. Your points make sense.
I was tempted by the weight savings and the possibility of better braking.
 

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I've had all the systems on my Dart. 10.87, 11.75, 13" Cobra and 14.25" SRT setups. I've always used the disc/drum valve only. No adjustable valves.

I've also measured the brake bias in terms of fluid pressure to the wheels, front and rear. The fluid pressure to the rear is 50% using the popular 1.125" plastic master.

I ran the dual diaphragm A body specific booster and has 13" idle vacuum. This was only with the disc/drum combos. The braking performance sucked.

I finally ponied up for the 13" front, 11.75" and a hydroboost. Now you are talking in terms of modern braking. I've done many track events this way with enormous success. After measuring fluid pressure to the front, I was not able to exceed 900 psi, until I installed the hydro unit.

After several years and events with the 13" fronts, I went to 14"/4 piston Brembo fronts. I also run pretty high end Carbotech pads at the track. The braking is phenominal. The thing about larger rotors and multi pistons, is it creates control. Standard systems are on and off.

Furthermore, for guys who insist on factory based systems, we offer upgrade kits featuring carbon kevlar pads, shoes, ss braided hoses and a real solid dot 4 fluid
 

mgoblue9798

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Thank you for responding. Your points make sense.
I was tempted by the weight savings and the possibility of better braking.
Not trying to talk you into or out of anything. I just like to get the most out of what I have already before deciding whether or not to spend a buck. Keep us posted on how your brake project turns out.
 

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Not trying to talk you into or out of anything. I just like to get the most out of what I have already before deciding whether or not to spend a buck. Keep us posted on how your brake project turns out.
I agree with maximizing your current situation, however, your comment about 1” larger rotors is very misleading for those without experience with larger brake systems.
Assuming there is correct line pressure, the difference in performance from the 11.75/single piston to the 13”/2 piston is huge.
 

Kern Dog

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I understand the principles of leverage. I would expect the 13” brakes to provide better braking. I’d love to see a comparison though.
I recall a Mopar Action article many years ago where Rick put a Viper caliper on one front side and left the iron single piston caliper on the other simply for testing purposes. The car didn’t pull to the Viper side from what I recall. That was with same size rotors on both sides though.
 
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BergmanAutoCraft

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I understand the principles of leverage. I would expect the 13” brakes to provide better braking. I’d love to see a comparison though.
I recall a Mopar Action article many years ago where Rick put a Viper caliper on one front side and left the iron single piston caliper on the other simply for testing purposes. The car didn’t pull to the Biper side from what I recall. That was with same size rotors on both sides though.
I can only speak from experience. And with that, whatever test Rick did sounds pretty irrelevant to proving high speed, repeated braking power, heat dissipation and modulation
 

mgoblue9798

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I can only speak from experience. And with that, whatever test Rick did sounds pretty irrelevant to proving high speed, repeated braking power, heat dissipation and modulation

I agree with maximizing your current situation, however, your comment about 1” larger rotors is very misleading for those without experience with larger brake systems.
Assuming there is correct line pressure, the difference in performance from the 11.75/single piston to the 13”/2 piston is huge.
You are of course correct.

Thing is how many of our weekend cruisers need this rather than just correcting and fine tuning a setup with mismatched parts?

It is all a matter of how you use the car I guess.

The bigger rotor will provide a larger heat sink, longer lever, and better overall performance if that is what is needed.

No intention trying to mislead anyone. Just trying to maybe help a mopar brother save a buck.
 

BergmanAutoCraft

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You are of course correct.

Thing is how many of our weekend cruisers need this rather than just correcting and fine tuning a setup with mismatched parts?

It is all a matter of how you use the car I guess.

The bigger rotor will provide a larger heat sink, longer lever, and better overall performance if that is what is needed.

No intention trying to mislead anyone. Just trying to maybe help a mopar brother save a buck.
I think the big thing people miss when they theorize about large brakes, is the ability to module a stop. Larger, multipiston brakes offer much more control which equals safety on the street. Why do we see so many photos of wrecked muscle cars these days? I believe one of the factors is poor brake performance. Today's world especially, demands better performance from our classics to stay out of harms way. Spending good money on brakes isn't a sexy choice, so it's often overlooked. Just my .02
 

mgoblue9798

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I think the big thing people miss when they theorize about large brakes, is the ability to module a stop. Larger, multipiston brakes offer much more control which equals safety on the street. Why do we see so many photos of wrecked muscle cars these days? I believe one of the factors is poor brake performance. Today's world especially, demands better performance from our classics to stay out of harms way. Spending good money on brakes isn't a sexy choice, so it's often overlooked. Just my .02
Again nothing wrong with what you have said.

I switched my D100 over to 3/4 ton just to get the bigger brakes as I needed them for towing.

I am also building an old Dodge RV that came with 4 whl 12x3 drums. No way was I putting that on the road driving in the mountains towing a toad behind it. Retrofitting a hydroboost system with 14" rotors, dual piston calipers, and 15x5 inch drums out back. Still in progress but no doubt will be a night and day difference.

FWIW the number one link among wrecked cars I see as an auto appraiser is chit tires. Newer cars with less than 4/32 tread that hydroplane, and older cars that have rock hard out of date tires. Both of course limit any braking that may have helped avoid the wreck.
 

Don67Satellite

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These are what Cass told me to buy. They are the cheapest at Rock Auto if you can add them to another order. I have a 15/16" aluminum masters on all 3 of my cars, and they all leaked. Put these Dorman caps and seals on, and knock on wood, they have been fine.

View attachment 1295773

Thanks for this @hunt2elk, I got both of those today and it was clear from just putting them on that they seal far better and tighter. The stock caps looked almost identical, but they were "sloppy" and the inside gasket "weak" I guess you could say. I had experienced some annoying minor leaking. I also did get clarification from Cass, for anyone else reading this, that "fill to the bottom of rings" on the side of the master means to fill the reservoirs right up to the bottom of the filler holes.
 
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