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Old 09-19-2012, 12:01 AM
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Manual setup. Worked, now doesn't.

Hello all. I lurk here a lot, and have gathered a lot of great information but now I have a specific issue that I need help with.

I successfully modified a car with a manual brake setup.
Modified the brake pedal for a 6.28:1 pedal ratio.
Used a wilwood tandem master cylinder 7/8" bore.
Calipers were stock (front disc)
rear wheel cylinders were stock (rear drum)
I had a 10lbs residual pressure valve for the rear drums
I had a metering/ hold off valve for the front calipers
I had a proportioning valve for the rear
I made all the lines were 3/16" hard line with a few stainless steel braided sections.

the car stopped well enough for me to feel comfortable driving it. Manual feels different from power, but I could lock the wheels easily.

that car is now stripped, and I wanted to bring the manual setup on to my current car for the sake of simplicity.

same pedal ratio
same master cylinder
same front calipers
rear is disc
NO residual pressure valve for the rear
NO metering/ hold off valve for the fronts
I have a proportioning valve for the rear calipers
I made all the lines... again, 3/16" steel lines with a few braided sections.


I have bled the system a hundred times. I even pulled the MC and bench bled it and reinstalled in the car, and re-bled the car. Same crap.


The pedal feels a bit spongy at first as I take up system slack, then it gets firm and I can't push it anymore about 3" from the floor.

the car slows, but does not come to a screeching halt even at very low speeds when stomping as hard as I possibly can on the pedal.

the pedal does not sink when holding it
I've bled the system many many times. 2 man method. Bench bled MC. Vacuum pump method. 1 man method. There is no way air is in the lines.

I'm 99% sure I have no leaks (pedal does not sink when constant pressure is applied for long periods of time)

The MC is practically brand new. I've had it for a few years, but it has fewer than 100 miles on it.

I am completely stumped as to why I cannot get the setup working. I'm assuming it may be related to switching from a disc/drum car to a disc/disc car, but if anything, increasing cylinder bore size at the caliper should INCREASE brake torque and DECREASE pedal effort... which is the opposite of what I'm currently experiencing.

any help or insight will be greatly appreciated. I'm calling wilwood tomorrow for technical advice also.

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Old 09-19-2012, 10:41 AM
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Welcome to the forum!!
Are you using the old disc/drum m/c?
Might need a disc/disc one.
Here's a link for trouble shooting.
Classic Chevy, Chevrolet, GMC, Ford technical articles
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Old 09-20-2012, 12:38 AM
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the MC I'm using is a 7/8" bore tandem wilwood unit, marketed as being both for disc/disc and disc/drum setups.

I called wilwood today, and didn't get any definitive answers. The tech assures me that a 6.28:1 pedal ratio with a 7/8" bore MC should not have any issues generating the pressure required to actuate my calipers with authority. I bounced some ideas off of him, but we were talking it circles.

It appears I'm on my own to troubleshoot the system.

I re-read the directions on my mityvac 8500 pump today, and vacuum bled the brakes again. This time, I am 100% sure I have bled the system properly.

the pedal feels nice and consistent, but alas, I'm still not getting the stopping power I feel the car SHOULD have.


my next course of action is to replace the front pads (I'm hoping they're horribly glazed from sitting outside, or got contaminated from the ridiculous amounts of rust on the rotors) and that it will be the magic thing to cure my brake woes.

If this doesn't work, I'm going to get a brake pressure gauge and go from there.

If my pressure is low, I'm going to change the system from a single tandem MC to a dual MC setup.

if the pressure is ok, I'm going to upgrade the calipers with calipers that have a larger piston area.
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Old 09-20-2012, 08:18 PM
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What kinda car are you trying to stop. Is it hearier then the other car. Bigger weight means bigger brakes.
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Old 09-24-2012, 03:50 PM
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Quote:
Originally Posted by Cape Cod Bob View Post
What kinda car are you trying to stop. Is it hearier then the other car. Bigger weight means bigger brakes.
both cars are the same make/model. the only difference is the years. both are similar trim level, so weight is within 100lbs of one another... another big reason why I was so darn confused.

what turned out being the problem was just badly glazed rotors and pads. I swapped the rotors because they were pretty much solid rust. That helped pedal feel, but I guess the pads were contaminated or otherwise destroyed from sitting so long (they were super cheap pads)

swapped the pads and now the setup works just as I remember it.

When I upgrade the front brakes to the wilwoods, I'll not only be going from a 10.8" rotor to a 12.19" rotor, but I'm also going to up my calipers from 3.54in^2 to 4.80in^2 to help improve my ability to stop in a panic.

glad I finally got this sorted out =)
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