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Discussion Starter · #1 · (Edited)
First off, I've searched and found nothing but if anyone has a better search keyword and this info exists, let me know.

We're trying to eliminate the power brake booster on my brother's buggy. To be precise, it's a chassis now so we're trying to eliminate the NEED for a booster.
- Front brakes will be stock '85 ford 1-ton, huge calipers, stock brackets and rotors. Hopefully that's one system we won't have to mess with.

- Rear brakes will be 3/4T GM fronts on a 14 bolt.

The idea here is to use a CNC, Wilwood, or whatever dual piston master with pedal mounting brackets and all.

Question is, what size masters to use and will this even work? Has anyone tried it?
My best guess is a 7/8" bore for both and the balance bar will probably be biased toward the front a little. CNC apparently has a 8.7:1 pedal that will get the force on the pushrods up pretty high and with the relatively small bore I think it'll have decent line pressure (700 PSI or so) but I'm a little concerned about the pedal travel with the large calipers. We can go with a 1" bore master but then we lose some of the line pressure.

Anyone have constructive input and/or some experience with this situation?

Vehicle specs:
We're aiming for about 4500#, 110" wheelbase, 42" TSL's and 45-50% rear weight. Height should be fairly low, drivetrain centerline should be about 31-32" off the ground with the 42's mounted.
 

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Sounds about right to me... dual 7/8".. and that big pedal will be a plus!
 

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Discussion Starter · #4 ·
Glenn, how is your pedal effort? How is the travel? Does it stop you like you want?
The Wilwood catalog says you have a 7:1 pedal so I'm thinking if the pedal travel and feel are OK, we might need to look into 1" bores rather than 7/8" since we can get a bigger pedal ratio if needed and will have bigger calipers on the front. We might end up spongy if we go with a 7/8" bore.
We're trying to avoid the $100.00 "learn it yourself" fee (replacing both masters if we really mess up) by hopefully getting it right the first time so thank you for your input!
 

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To get 700 PSI in the lines you need to push on the pedal with 97 lbf. I don't think you want to push that hard to lock up the brakes. It is no problem to stop a big truck with manual brakes, and like you seem to know a smaller MC or a longer pedal will give more braking force, the problem is if you go to small on the MC bore you may not have enough fluid capacity for the big 1 ton calipers, This is why things like trophy trucks fab big long brake pedals.
 

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Damn you are spending way mor than you need to. In my Flatty with D60's front and rear, I was stopping on a dime using a Master out of a F350. Just get any master out of an F350 and slap it in there. I was stopping the Flatty with D60's and 38's with no problems. Hell it almost had too much stopping power.

Dimitri
 

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Stephen, I used two 7/8 master cylinders for my Ford buggy. It weighs 3800 -4000 lbs. & is usually 1000 lbs heavier in front than back. It uses monster 2- piston (stock 3/4 ton) calipers in front and 11 x 2 drums in back, with a knob style proportioning valve. I should have gone with a 3/4 bore or higher leverage ratio .Due to space limitations longer pedals isn't possible, though I will probably redo the mount to change the ratio.(it's 6.2 :1). At the time that I built it I was after a nice firm pedal so I went with the fairly large M/C but it's awfully tough to hold the rig on a steep downhill with the tranny in drive.

On the next rig I used a single long travel Wilwood 7/8 M/C and bumped the leverage ratio to 8 :1. It has GM 1/2 ton calipers in front and 11" x 2" drums in back. It weighs 3100 lbs, the front is 600 # heavier than the rear. This one doesn't have a prop valve (& doesn't need it) but has Jamar cutting brakes. The pedal is a little spongier but it definately holds better than the last one. And with the long travel m/c there's really no danger of running out of travel. However, on your application with the better braking power in back (without knowing the weight bias & COG) I'm guessing it probably would lock the rears up too soon without some sort of prop. valve.

IIRC, the master cylinders have a design limitation, as to the leverage, and overall force, that should be applied to them.

It's hard to know whether or not the max. recommended leverage will give you enough braking power without actually trying it, but based on my experience, 7/8 m/c's are too big if using two with no assist. (That's actually double the area and half the power of alot of factory 4x4's) 3/4 would work excellent, I think, if you use the long travel models. BTW, two 5/8 m/c's have the same area as a single 7/8 m/c.

Peter
 

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i am also gettin rid of my booster and switching to dual masters.

gonna use a cnc hanging pedal assembly. probally around 7 or 8 inch drop with dual 3/4 masters. they will be pushing 4 pistion willwood dynalight calipers.

i was in a quandy about the setup as well and then i was talking to tracy jordan at put up or shut up and noticed that was the same same setup he was running. asked him how they worked and after some chit chat about it decided to run it. you can also check with mike shaffer at shaffer off road as i belive that is the same setup he had on the old rig and he is also a willwood dealer. give him a call 775-885-9944
 

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Discussion Starter · #9 ·
I'll have to check the space available under the dash in this thing to see if we can get the higher ratio pedals in there. I was guessing the 8.7:1 ratio with 7/8" bores would be a good starting point and that sounds approximately right judging from what everyone is saying here. But they may not fit.

Assuming 45 lbF pedal pressure on front M/C (figuring on 75 lb or so on pedal with bias bar sending a bit more force to the front):
8.7:1 pedal ratio with 7/8" bore gives 651 psi
6.25:1 pedal ratio with 3/4" bore gives 636 psi

Either one is going to act basically the same as far as line pressure and pedal travel is concerned, I think it comes down to space available to choose between them.

If anyone has any more experience to throw in here, it's sure welcome.

Camo, are you using the 4 x 1.75" piston calipers? On all 4 wheels?
 

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A little different aplication but I'm using 3" bore chevy calipers on my pinion brakes with a single 3/4" bore wilwood and the stock power brake pedal and it works great. Pedal pressure is light with excellent feel. I am running 2 of these calipers with this master cylinder on a 5500# truck also ran this set up when the truck weighed in at 6800# with the same results.
 
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