Master cylinder bore size - Chevy Nova Forum
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Old 15th-May-2019, 12:08 AM   #1
bulldoguk
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Master cylinder bore size

I need to replace my master cylinder and booster. I have factory front discs but I’d like to swap the rear drums to discs either now or later on. Im confused as to what bore size I need, 1” or 1.125”

Help! Lol
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Old 15th-May-2019, 07:23 AM   #2
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I went through the same confusion when I did the rear disc conversion on my car. I'm no expert, but here's what I discovered:

Cars with factory front discs had the 1.125" bore MC. The Corvette (and I *think*) all-drum cars had the 1" bore. My understanding is that the smaller the bore, the more "punch" the brakes have. The Corvette got the smaller bore because it was a sports car, despite having 4-wheel discs. The all-drum cars got the 1" bore because they needed the extra umph.

So what happens if you go with a 1" bore when converting to all discs? I went with the 1" bore because it was the only size available with the hardware I chose and, in my experience, I couldn't tell any difference. The only caveat here is that I hadn't driven the Nova in so many years, and I was so used to driving modern cars with better brakes that maybe it really does stop better with the 1" bore. But I don't think there's much difference, if any, and I wouldn't expect you to have any trouble if you go with 1" bore. In fact, I'd probably recommend it.

Just remember that you have to change out the proportioning valve for an all-disc setup no matter what size MC bore you go with, but this is no big deal.
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Old 15th-May-2019, 09:24 AM   #3
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I don’t have power brakes. I do have 4 wheel disc brakes. Wilwood 6 piston calipers up front and stock Ford Explorer calipers in the rear. I run a wilwood 7/8” bore master cylinder. Brake pedal feels great. Stops on a dime...
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Old 15th-May-2019, 10:39 AM   #4
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I gather that the smaller the bore the higher the pressure but the larger the more the more flow, so it comes down to a trade off between pedal travel and pressure (from your foot) needed to actuate the brakes. It probably won’t matter which one I guess, as I’ve never driven the car with brakes lol.

For an extra $300 I’m tempted to do the rear disc conversion now rather than later. I have 16” Buick wheels on it so they will fit over the calipers...
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Old 15th-May-2019, 01:00 PM   #5
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As Tony says; the smaller the bore the longer the throw (and easier the push).
I don't like power brakes in my hobby cars mostly because I don't like the look of the booster under the hood.
In this case you need to be careful when sizing the MC bore. An eighth inch makes a big difference. Somewhere between 7/8 and one inch is a typical range depending on the ratio of the brake pedal and the size of the calipers. Wilwood has a good online calculator that takes everything in to consideration.
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Old 15th-May-2019, 05:22 PM   #6
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Quote:
Originally Posted by Alf View Post
depending on the ratio of the brake pedal and the size of the calipers.
A LOT of people do not take this into consideration when choosing a master cylinder. Do not assume when dealing with a possible life saving part. If you don't understand how to find the brake pedal ratio call Wilwood, they are great with helping.

Mike
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Old 15th-May-2019, 05:28 PM   #7
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All 67-68 Novas had a 1" bore master cylinder whether drum or disc. Not sure on 69-70 but I think they are the same as 67-68. 71 up have 1-1/8" bore if disc, 1" if drum.

In general you size the master cylinder bore to the front calipers. Most GM cars use a 1-1/8" master cylinder with discs and a booster and 1" with no booster - when you use the early 70s GM calipers. Wilwood calipers, even the 6 piston ones, have a smaller overall piston size so they use a smaller master cylinder - 7/8" for manual brakes, and 15/16" or 1" with a booster.

As already said, a smaller master cylinder bore makes more line pressure but the pedal also has more travel.

Keep in mind too that the pedal ratio changes when going from manual to power. Manual is 6:1, and power is 4:1. Higher numerical pedal ratio is more line pressure and more travel.

Late model cars use big bores, boost, and low numerical pedal ratio. The pedal hardly moves before braking happens. If that's what you want, go that direction.
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Old 15th-May-2019, 06:26 PM   #8
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I have factory front discs in my 72 SS so I guess I’ll be going with the 1.125” bore. 1” and 1.125” are the only options from the manufacturer and these are not high performance big brakes, I don’t have the money for those lol. Thanks gents
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Old 15th-May-2019, 06:54 PM   #9
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Quote:
Originally Posted by Rick_L View Post
All 67-68 Novas had a 1" bore master cylinder whether drum or disc. Not sure on 69-70 but I think they are the same as 67-68. 71 up have 1-1/8" bore if disc, 1" if drum.

In general you size the master cylinder bore to the front calipers. Most GM cars use a 1-1/8" master cylinder with discs and a booster and 1" with no booster - when you use the early 70s GM calipers. Wilwood calipers, even the 6 piston ones, have a smaller overall piston size so they use a smaller master cylinder - 7/8" for manual brakes, and 15/16" or 1" with a booster.

As already said, a smaller master cylinder bore makes more line pressure but the pedal also has more travel.

Keep in mind too that the pedal ratio changes when going from manual to power. Manual is 6:1, and power is 4:1. Higher numerical pedal ratio is more line pressure and more travel.

Late model cars use big bores, boost, and low numerical pedal ratio. The pedal hardly moves before braking happens. If that's what you want, go that direction.
This is awesome information - kind of the opposite of what I thought. Thanks for posting.
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