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Discussion Starter #1
Hey fella's

I have a 71 with stock power discs that I'm switching to Wilwood discs and I would like to switch to their master cylinder. I am running a big solid roller cam and don't get much vacuum to run power brakes. My question is can I simply bolt in the new Wilwood master cylinder in place of the old m/c and booster? Does anything else need to be changed?

Thanks in advance for the help
 

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Just their bottom of the line alum body with plastic reservoir, around $120. I also have no idea what bore diameter to use.
 

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I have the same dilemma and since my car is not finished I cannot advise you. However, I can give you the logic I used.

The basic theory is that every pound of pressure produced by the master cylinder piston is applied to every square inch of caliper piston. So if you create 100 pounds of hydraulic pressure with a 1” bore master cylinder and you have a four square inch caliper piston, you generate 400 pounds of force against the rotor. So to find the multiplier, divide the area of the piston(s) in one caliper by the master cylinder bore diameter. In this case, 4 divided by 1 is 4 so the amount of pressure your right leg can produce is multiplied by 4 at the caliper. Using this same example, if you had a 1/2" bore master cylinder, 4 divided by 1/2 would be 8 so you would produce 800 pounds of force at the caliper.

This pressure multiplication does not come without a cost however. Since the smaller bore in the master cylinder is moving less fluid for every inch of travel, the pedal will have to move farther to produce the same results. Think of it as a hydraulic cheater bar. Speaking of cheater bars, you can also move the push rod higher on the pedal to create additional mechanical advantage or lower it to decrease the pedal travel. In fact, the factory has two holes in the pedal already. The lower one is for power brakes and the upper one is for manual brakes.

All that said, wanting to get the best performance out of my manual brake system, I have four piston Wilwood brakes on all four corners and their aluminum master cylinder with a 7/8” bore. I didn’t make my decision based on any calculations because I had the master and later decided to get the disc brakes to go with it. I feel that this will allow me to generate the greatest amount of hydraulic pressure although I may not like the pedal travel it produces. If I don’t like it I will probably call Wilwood for advice before I make any changes. In the end the worst that can happen is I change the master to a 1” bore or drill a new hole in the pedal to tune the system to my liking.

Common master cylinder bore sizes are 1-1/8” for most power brakes, 1” and 7/8” for manual brakes.

Hope this helps,

Steve
 
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