Master cylinder bore diameter?
 

20 years ago I installed 4 wheel wilwood disk brakes on my 67. The light weight drag racing type. I also installed a manual wilwood master cylinder with 1 1/6" bore. Always had a lot of pedal effort to stop, but got use to it. I was reading an article that you should use a smaller bore for manual brakes. Do you think going 15/16 or 7/8 will make much of a difference? What bore diameter are you using for manual brakes?

Small bore will be a nicer feel.

My 68 GTO had great pedal feel with low engine vac: G-body MS into 82-86 F-body rear calipers.

What size do you have now, 11/16”?

Agree. Mike - You might mention if you have any rear proportioning, too.

K

Please report back what Wilwood recommends.

I'm in the middle of this as well.

K

Back when I was converting my 1964 Tempest to manual C6 Corvette Z06 4-wheel disc brakes, Tobin at KORE3 who I purchased the brake kit from recommend using either a 15/16” bore GM or a Wilwood 7/8” master.

I ended up going with the 15/16” GM master (1977 Malibu), mostly to save money and to be able to readily source locally a replacement master if needed. I did install a Wilwood adjustable proportioning valve in the rear brake circuit. Stalled project so I can’t tell you how it worked but I’m confident it’ll be right.

On my 1964 Le Mans convertible I’m using a 1” bore GM master (1970 Firebird) with factory 1969 GP 11” front discs and 9-1/2” rear drums. I was replacing a leaky 1-1/8” master and the softer pedal with slightly increased travel made stopping much easier to modulate with reduced effort. Manual brakes as well.

Original 68 Firebird with manual brakes is 1” for what it’s worth.

tuning in....ive got the light weight willwoods on my 65...4 wheel disc. ive also got a 08 c6z master on the shelf....wondered about using that cylinder with an adjustable pv as a manual brake set up

Has the pedal been changed? Or know what the pedal ratio is?
Pedal ratio has a lot to do with the brake application.


Then the Master cyl area and the caliper piston area needs to be considered.


:confused:

The brake pedal has 2 different ratios built into the design.

More leverage for manual brakes, with the pushrod clevis in the upper hole.

Less leverage for power brakes using the lower hole in the pedal arm.

The mounting angle of each type, manual master or master with booster dictates which hole you use. This makes it so you can’t use the wrong pedal ratio as designed.

I'm using a 7/8" Wilwood with GM 12" B-body discs and drum rears. It works/feels great. I had a Strange 15/16" that started leaking. Replaced it with the 7/8" Wilwood. I was surprised that the feel was noticeable and better. So, a small change does make a difference. This is a manual setup of course.

Do you know your pedal ratio?

K

Measured pedal ratio, 6-1. Pedal is original to the car so I am guessing that is the original manual pedal ratio. Looked on wilwoods pedal ratio chart my current 1 1/16" bore equals 674 psi. 15/16 = 869, 7/8 = 1000.
Have an e-mail into wilwood we'll see what they reply.
I am thinking 15/16, that would be about a 200psi increase.

I do have a rear proportioning valve, open up all the way.

It's the factory top/manual hole. I'd have to measure it to tell the ratio.

Probably like Mike S stated (6:1ish).

For manual brakes, a 15/16" or 7/8 would be better. 1-1/16" is certainly used for power brake use.

George

For the IA Camaro I have manual broakes Wilwood DYnalites up front rears, 2.5" GM metric on 79-81 TA brackets. Moved pedal to the top manual hole for ratio. Started with the Strange/Mopar 1 1/16. A LOT of pedal pressure I could see the dash and firewall move! I have brake pressure gauges front and rear. I used a similar Mopar smaller than 1" as a 1" still had a lot of pedal needed. I want to think a station wagon MC. I have 3 different sizes al bench bled with plugs where the lines are. I have a slightly smaller one ready if I get tired of this one.

Got a reply from wilwood. He recommends 15/16 bore master cylinder. That what they recommend for my calibers. He Said it would be over 20% less effort than my 1 1/16. He said the 7/8 would make the brakes lock up to easily. He had to remind me my brakes are for drag racing and will overheat on the street because of the thin rotors.

Installed the new wilwood 15/16 master cylinder. Defiantly less effort to stop. Can actually lock them up with one foot instead of 2. Down side is there is more pedal travel but can live with it.