Master cylinder bore size question

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69ss
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Master cylinder bore size question

Post by 69ss » Thu Aug 30, 2018 10:41 pm

Hello, I have a 69 Chevelle with the stock power disc/drum system. Stock uses an 1-1/8 bore master cylinder, I had a brand new Wilwood master cylinder given to me but has a 1inch bore. What kind of difference in pedal feel do you think the smaller bore would make if I were to go with the Wilwood master cylinder? Thanks for any input!

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Re: Master cylinder bore size question

Post by John Wallace » Fri Aug 31, 2018 8:01 am

The Wilwood would give higher pressure if no other changes.
But will the smaller volume of brake fluid be enough to activate the brakes?
(disk brakes or drum?)

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Re: Master cylinder bore size question

Post by 69ss » Fri Aug 31, 2018 8:37 am

disc front, drum rear, power brakes

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Re: Master cylinder bore size question

Post by John Wallace » Fri Aug 31, 2018 11:57 am

Your master cylinder now isn't a dual chamber one?
(would have 2 different sizes for front and back)

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Re: Master cylinder bore size question

Post by 69ss » Fri Aug 31, 2018 12:06 pm

Yes, it's a dual chamber. The whole sysrem is the factory 69 Chevelle power front disc/rear drum system

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Re: Master cylinder bore size question

Post by MadBill » Sun Sep 02, 2018 7:05 pm

69ss wrote:
Thu Aug 30, 2018 10:41 pm
Hello, I have a 69 Chevelle with the stock power disc/drum system. Stock uses an 1-1/8 bore master cylinder, I had a brand new Wilwood master cylinder given to me but has a 1inch bore. What kind of difference in pedal feel do you think the smaller bore would make if I were to go with the Wilwood master cylinder? Thanks for any input!
The pedal pressure for a given decel rate will go down by ~ 12% and the pedal travel will increase in the same proportion.
Felix, qui potuit rerum cognscere causas.

Happy is he who can discover the cause of things.

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Re: Master cylinder bore size question

Post by rustbucket79 » Sun Sep 02, 2018 10:17 pm

Running a 7/8 bore Wilwod master on my 79 Camaro factory disk/drum setup. Pressure isn't a problem, volume is. You need to pump the brakes once to push out the rear shoes, then the second pump you are good to go up to 1200 psi. Going to rear disk for next season, hoping to eliminate that first pump.

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Re: Master cylinder bore size question

Post by rustbucket79 » Sun Sep 02, 2018 10:19 pm

Pedal effort is greatly reduced. Haven't run the car for a few years, but I believe effort is similar to power brakes with great vacuum.

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Re: Master cylinder bore size question

Post by MadBill » Sun Sep 02, 2018 11:58 pm

MadBill wrote:
Sun Sep 02, 2018 7:05 pm
69ss wrote:
Thu Aug 30, 2018 10:41 pm
Hello, I have a 69 Chevelle with the stock power disc/drum system. Stock uses an 1-1/8 bore master cylinder, I had a brand new Wilwood master cylinder given to me but has a 1inch bore. What kind of difference in pedal feel do you think the smaller bore would make if I were to go with the Wilwood master cylinder? Thanks for any input!
The pedal pressure for a given decel rate will go down by ~ 12% and the pedal travel will increase in the same proportion.
Err... make that ~27% :oops:
Felix, qui potuit rerum cognscere causas.

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Re: Master cylinder bore size question

Post by pdq67 » Mon Sep 03, 2018 9:24 pm

I have a really old mag with an article in the very back of it that David Freiburger wrote. He put a 1.25" piston dia. M/C on a Chevelle that had been converted both front and rear to disc brakes. He said it worked great. I figure it was a volume thing...

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