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Master for 4 wheel disc?

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Mr_Roboto View Drop Down
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    Posted: Sep/24/2022 at 7:42pm
Trying to figure out what master I need for 4 wheel disc on an 82 Eagle. I am going to the Liberty rear Brakes on an 8.25 (mostly because I'm not a fan of 2.34 gears!) Anyone got any ideas? I have read it's a "ford style" booster but am not sure if I can just grab a master cylinder and go. If worse comes to worse I may convert to a GM Hydro boost (I have one here,) but would rather pass on it for now.

Also, I'm presuming my stock "power brakes" rod ratio is good to go? I think I calculated it to be something like 2.77!

I'm going to put those in, throw in new front hoses and redo pads on everything as well. I may also install my line lock, but am not sure.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote nickleone Quote  Post ReplyReply Direct Link To This Post Posted: Sep/24/2022 at 8:15pm
nick
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Mr_Roboto View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Mr_Roboto Quote  Post ReplyReply Direct Link To This Post Posted: Sep/25/2022 at 1:00am
I've read the article before and crunched the math out and I end up with the following based off the parts I have:

MC area 0.69 in^2 (This is for each section, it's a dual bore master cylinder)
FR Caliper Area 5.3 in^2 per caliper
R Caliper Area 2.8 in^2 per caliper
2.85:1 pedal ratio (10/3.5in)

Lets say we have 100lbs of pedal force we end up with:
100*2.85=285lbs force at the MC
285/2=142.5lbs force for each section
5.3*2*145 (doubled because there's 2 calipers)=1537 lbs brake force for the front
2.8*2*145=812lbs brake force for the rear

Does this seem correct? Also, is this force reading desirable? I don't think it's getting to the numbers themselves I find hard, it's the point of what numbers do you actually desire to have in the first place? Overall it seems pretty close to me in terms of what I should be shooting for front to rear bias wise. 

Besides that the other question is lets say I want to change bore size to a 7/8 or 1" bore MC instead of the 15/16. Are there any OEM options that bolt on? Heck would I be better off to drill a hole between the power brakes and manual brakes hole and adjust the ratio some that way if I need to?

This also ignores that I have a booster. I'm pulling 20inhg currently and according to Rockauto the booster has a diaphragm radius of 8.93". That comes out to:
(8.93/2)^2*3.14159*(20/29)*14.7=193.16lbs force that would be added to the 285lbs above.

Does that seem correct?


Edited by Mr_Roboto - Sep/25/2022 at 1:09am
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Trader View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: Sep/25/2022 at 11:46am
I believe your way overthinking this. The factory master cylinder as you point out, has plenty of pressure to operate disc brakes.
Many have made this conversion using the factory MC with no issues.
Upgrade to the disc brakes with the factory MC you have and drive the car for a while. If you feel there is need for a change, then you will have a better idea if you want a smaller or larger bore MC.
My guess is your going to be perfectly happy as is. 
The only thing that would be a caution is the extra rear braking power. You do not want the rear brakes to lock first. You may need an adjustable pressure regulator for the rear to reduce that braking force.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tufcj Quote  Post ReplyReply Direct Link To This Post Posted: Sep/25/2022 at 12:32pm
You might try keeping the master you have.   When I went to 4-wheel disc on my Jeep CJ, I used the disc/drum master, and even had to add a reducer valve to the rear because the rears would lock on a wet or snowy road.

Bob
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tomj View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tomj Quote  Post ReplyReply Direct Link To This Post Posted: Sep/25/2022 at 5:54pm
I agree with trader and Bob. 

And if it's not to your lokong (too much effort, too much travel) you'll have a baseline. 99% percent of master cyls are 1 inch bore. A 1/16" bore diam change is significant. Drum to disc doesn't change fluid volume much of at all in my limited experience. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Mr_Roboto Quote  Post ReplyReply Direct Link To This Post Posted: Sep/26/2022 at 8:40am
I'll probably end up leaving it. Considering I have to replace all the hoses as well (front ones are toast) I figured that now is the time to stick a master on since I have to go to the labor of bleeding the whole thing. 

I have a line lock I'll probably toss on and I have an adjustable prop valve for the rear I'll probably toss on as well. My Lemans required it when I went to front LS1 Camaro Calipers because the rear would lock up way early if I didn't. I do need to figure out where to stick those, that part of the under hood is pretty crowded honestly.
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