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Keane165's 1970 LeMans

2022 June
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I just ordered a front disk conversion for my 55 Chieftain from performance online but I got a new master cylinder that mounts to the firewall. I want to keep it the way it was mounted to the frame but I can’t find one that would fit. Does anyone have any suggestions?

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Welcome Pete. I have a question before I try to answer. Are you wanting to run a power brake booster mounted to the frame or are you willing to run without power brakes?

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On 8/26/2021 at 8:07 PM, Poncho pete said:

The picture of my existing master cylinder is from the bottom of the car looking up I’ll take a better picture tomorrow. Thanks

Pete, presuming you’ll remain interested in this conversation this will take several exchanges! 
I believe the bore size of the stock “55” master is 1” and quite obviously a single fluid line out! While I don’t know specifically what duel master cylinder you have I would encourage you to get one that has at least a 1” bore! Preferably a 1.125 bore, while this will give a little harder pedal if you change nothing else, I will give suggestions that will also alter that effect! Make sure the two line ports of the new master cylinder are not on the frame side. You will need to make or have made special fittings for those ports and also serve as mounting bolts for the new brackets that will have to be made.
So to be clear, that means you will need a distribution block that will mount somewhere else! This distribution block will take the single front line and divide it into two lines and the same for the rear line. 
Take a look at your brake pedal! Measure the distance from the master cylinder push rod attachment to the middle of the pedal, the pad you push with your foot! Than measure the distance from the center of pivot point of the pedal, where it mounts on the upper shaft to the master cylinder push rod! Note them and tell me what they are! 

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Everything is outside the firewall so the measurements are a little hard to take but this is what I got. First picture is from the 55 shop manual the second are the numbers I came up with. Thank you for taking your time to help me 

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59 minutes ago, Poncho pete said:

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Everything is outside the firewall so the measurements are a little hard to take but this is what I got. First picture is from the 55 shop manual the second are the numbers I came up with. Thank you for taking your time to help me 

Pete, the 1” bore is ok and may even work well for you! So since you have it at least start there, but you are staying with manual brakes and you are moving from drum to disc! Disc hands down out stop drum, but disc require more fluid volume & more applied pressure! So I mentioned a 1.125 piston master cylinder because that is what I ran on my 4 wheel disc that I ran on my “69” Z. Things can get pretty complicated with a brake system & there is a lot of math if you want an excellent stopping car!

Your pedal ratio is 4.6 to 1. You may need to go as high as  7 to 1 because the pedal is going to be harder just by moving to disc, but that can be addressed down the road. If you can take picture of the brake pedal assembly that shows me the entire pedal and linkage. As soon as I can I will sketch up a fitting that you’ll be able to get made that will serve as a fitting for the master cylinder and serve as a attaching bolt for the new bracket!  
 

You are most welcome!

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I can’t get a good picture. I will do power brakes if it’s easier. My thought was an L bracket they the master sits on with to ears hanging down that would mount to the frame 

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15 hours ago, Poncho pete said:

can’t get a good picture. I will do power brakes if it’s easier. My thought was an L bracket they the master sits on with to ears hanging down that would mount to the frame 

That’s good enough Pete! Not a lot you can do with that pedal setup without getting real involved! So let’s JustA stay with what you have for now. 
 

The bracket with the ears will work, but it will need to be made from at least 1/4” steel stock! For added security & strength! It should also be design with a gusset that runs from the ear arrangement to the vertical side that attaches to the frame! This will eliminates any flex. Remember you are going to be applying at least 500 pounds of pressure to the master cylinder! Maybe more in a panic stop! 
With the limitations you will have you need to be very selective in your pads and rotor selection!

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