Master Cylinders

Scott Newton · Posted: Thu Mar 17, 2011 8:33 am Post subject: Master Cylinders

The MC's on my car are well... ancient. Even though they still hold pressure, it's really about time I replace them.

As is nearly everyone - I'm using [the old-school JFZ equivalent of] Willwood DynaLight Single calipers, two front & one rear. I'm also going to be doing solo-only duty with this car.

What cylinder bores would everyone recommended for this setup?
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1989 Red Devil w/ Kawasaki 440 (solo only)

Chris Ross · Joined: 07 Jan 2008 Posts: 766

What bore are your current MC's? How is the brake bias in your car ... balanced, too much front, too much rear? If it is balanced, I would not change your MC bores. If the pedal is really light and hard to modulate, then you can go down in size on both MC's or if it is too hard then you can go up in size. However, it doesn't always work out to change both sizes up or down as the step sizes can be different. For example

Say you had a 0.75 inch MC on the rear and a 0.7 inch MC on the front and you wanted to decrease the MC size of both to reduce the amount of pedal force. The 0.75 inch rear would step down to a 0.7 inch and the 0.7 inch front would step down to a 0.625. The step size is different between your front and rears so you would end up increasing the brake pressures on the front, there by altering your brake bias. I have developed a set of calculations for picking brake components and it seems to reasonably accurate as I was pretty happy with the brake balance on my car and the bias adjuster is able to fine tune either the fronts or the rears locking up. Since you already have a brake setup on your car I would think back and evaluate how your brakes currently work. Answering the questions above can help provide direction as to what to do with the MC sizes and type.
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Scott Newton · Posted: Thu Mar 17, 2011 11:47 am Post subject:

I do have a brake setup on the car, but it left a LOT to be desired. My balance bar was bent, my pedal was binding, my MC's were leaking, my pads were glazed over, my rotors were warped and my tires were rock hard.

As it was, I had too much front bias. It would take me about half a straight to get the car slowed up for the next corner - any harder braking and I would lock up the front end. Given how many other issues there were though - I'm basically replacing the entire braking system for this year. The only parts I'm planning on keeping are the brake lines (as they're pretty new) and the calipers themselves (which I'm going to rebuild). As such - last years baseline probably isn't that valid anymore.

I got a suggestion to use 3/4" fronts and 7/8" on the rear. Does this sound like a good ballpark for most people?
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1989 Red Devil w/ Kawasaki 440 (solo only)

Chris Ross · Joined: 07 Jan 2008 Posts: 766

It depends on your rotor diameters and your piston diameters. Fill in the blanks below and I'll punch them into my calculator and give your a rough idea on a MC setup.

Correct me if I am wrong but so far I understand your setup to be as follows.

Front
Dynalite Replica 1.75" piston per side
Rotor Diameter:

Rear
Single Dynalite Replica 1.75" piston
Rotor Diameter:

Wheelbase:
Weight on front axle:
Weight on rear axle:
Tire Diameter: 19.5 or 18
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Scott Newton · Posted: Fri Apr 01, 2011 4:59 pm Post subject:

Sorry for the delay (I didn't actually have my car out of storage when I first posted this), but here are your answers: