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Brake dual master cylinder fun facts


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I sat in on the brake tech session at the Cazenovia Trek and just now got around to investigating some questions I had on brake master cylinders. The tech subject was a presentation on conversion from single to dual M/C performed and the issues with getting a firm pedal after installing a dual master cylinder. The presenter recalled that the master cylinder was listed as an early Corvette master cylinder. In my quick Google search, I found that the Corvette made 4 wheel disc brakes available starting on the 1965 model. I also found that Corvette transitioned to dual master cylinder in 1965 (for power brake cars only) then in 1966 all Corvettes were dual master cylinder.

So if all the above is accurate, then the presenter used a front disc/rear disc brake master cylinder in his conversion.

Now we look at the difference between disc and drum master cylinders. The differences seem to boil down to this:

Disc:

Larger reservoir

Longer stroke

No residual pressure valve

Drum:

Smaller reservoir

Shorter stroke

Residual pressure valve (typically 10psi)

So, when selecting a master cylinder, the simplest conversion would be from a car with four wheel drums. The next choice would be from a car with four wheel disc brakes then add external residual pressure valves. However, from the differences noted above, I would not recommend a M/C from a donor car with front disc/rear drum brakes. The difference in fluid delivery and pressure in the two circuits would cause two ends (front to rear) the Franklin drum/drum brakes to be unbalanced.

Since I don't even own a Franklin and dad's car is mechanical brakes and I have a race car to get ready to race this weekend, I'll leave the research into the ideal M/C up to someone else.

Cheers,

Jim

For a better summary on master cylinders see:

http://deemaxxcomponents.com/TechFull.aspx?ID=281

Edited by jimeby (see edit history)
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The presenter mentioned that the one he used was a 1 1/8" bore, which would make it the one used in the power-brake Corvettes from 67-75, (as well as many other Chevys of the era). That smaller bore gave him a little more braking force than the stock Franklin 1 1/4" cylinder, but means higher pressures in the lines than the original. Since it was a disc brake master cylinder, he had to use add-on residual pressure valves on the lines.

From what I've looked at, the simplest and most original-behaving conversion would be a master cylinder from a late-60's 4-wheel-drum 3/4 or 1 ton Dodge pickup, like the one used on the D300 from 67-69. It's a drum/drum setup and has a 1 1/4" bore. There's few "modern" cars that used that big of a bore - this seems to be the most common 1 1/4" bore application, and even it is starting to get hard to order. Mounting is very similar to the Chevy cylinder used by the presenter.

Making the slight shift down to the 1 1/8" bore, like the one he used, opens up a much more common GM master cylinder. 4 wheel drum Chevy/GMC 1-ton trucks from 67-72 had a 1 1/8" bore, and they're common enough that most chain auto stores should keep it in stock. This should give the same performance as the Corvette cylinder, but with the added convenience of built-in residual pressure valves.

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The master cylinder used by me, the presenter, was a GM style sometimes refered to as Corvette type master cylinder from Summitt Racing part number 760101-1 which as mentioned is very common and easy to find. However, I like the idea of a 1 1/4" bore with built in residual valves.

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