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Discussion Starter · #1 ·
I built a 4 wheel disc brake system on my 29 model A pickup, which stops adequately BUT I want the system to basically put me thru the windshield when I step on the brakes....can anyone suggest improvements to the basic set-up ???

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I know my first thread is about how my manual setup doesn't work (haha) but take this for what it's worth.

in all my research designing my custom brake setup on my car, there's a few things you can do to improve stopping power when you don't have a booster.

1) reduce the bore size of the MC

I don't know how you plumbed your setup, but if you're using a single tandem MC, most of them only go down to 7/8" bore. If you reduce the bore size of the MC, you increase the amount of pressure it can build (assuming you still have adequate volume. if your system is bled, and your pedal goes to the floor and you still aren't stopping, the MC is too small).

a dual MC setup will give you more flexibility in terms of picking a properly sized MC. I've seen these go down to 1/2" bore. Just be cautious with volume. if you're already using a 7/8" bore MC, a twin setup would let you step down to a 3/4" bore and increase your system pressure about 400psi given the same amount of effort.

here's some math I did when I was calculating my personal setup:

To figure how much pressure your master cylinder is putting out:
C = pedal ratio
D = pounds of pressure apply by your foot
E = area of you master cylinder
F = pounds of pressure out of the master cylinder
C X D / E = F

600 / .88354

MC area by bore size:
bore = area
5/8" = 0.30677in^2
3/4" = 0.44177in^2
13/16 " = 0.51845in^2
7/8" = 0.60132in^2

Area of 7/8 MC : 0.60132046885117136204949033508084

the following is pedal ratio x lbs of force on the pedal (I put 100pounds) and the pressure that should exist in the MC.

this is my personal setup
7/8 tandem MC setup with modified stock brake pedal:
6.28 x 100 / 0.60132 = 1044.36psi

calculations using a 6.25:1 Wilwood brake pedal p/n 340-5181.
7/8 twin MC
6.25 x 100 / 0.60132 = 1039.38psi

13/16 twin MC
6.25 x 100 / 0.51845 = 1205.51psi

3/4 twin MC
6.25 x 100 / 0.44177 = 1414.76psi

5/8 twin MC:
6.25 x 100 / 0.30677 = 2037.35psi

these calculations are with a 5.1:1 Wilwood brake pedal p/n 340-5180.
7/8 twin MC
5.1 x 100 / 0.60132 = 848.134psi

13/16 twin MC
5.1 x 100 / 0.51845 = 983.701psi

¾ twin MC
5.1 x 100 / 0.44177 = 1154.44psi

5/8 twin MC
5.1 x 100 / 0.30677 = 1662.48psi

2) increase the pedal ratio

the brake pedal is a lever arm. for manual setups, its said a 6:1 ratio is a great starting point.. but I've seen ratios anywhere between 5:1 and up to 7:1. You can increase the ratio of your pedal (this will reduce effort) but you will increase the throw (you have to push farther).

3) increase the cylinder bore size

increase the bore size of the calipers, and this will increase the amount of force the existing pressure can exert on the pads. you built your system, so you should know the piston area of your fronts and rears.

since majority of braking comes from the front, I'd look into possibly swapping to calipers with greater piston area just in front, and see if that helps getting more stopping power.

side note: I'm having issues with my manual disc/disc system also. It stops the car, but with no authority even at very low speeds. My pedal is a stocker I modified for 6.28:1 ratio, so that is fixed. I like my tandem MC, so I'm trying to leave that as is. I'm looking to increase the bore size of my calipers from 3.54in^2 to 4.80in^2 (the biggest calipers I can find that will fit my wilwood kit).

hopefully this helps. good luck man.
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