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master cylinder 1 1/8 inch bore


Ed Wieder

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Firm I purchased a rear disc brake setup for my 71 Monte said a 1 1/8 inch master cylinder is needed. Inline company, ebay seller, told me this upon receiving the goods and after saying his listing included all needed to convert to rear disc brakes. What difference, if any, would there be using a 1 inch bore master cylinder with disc/disc versus 1 1/8? Thanks, ejw71

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So last night I started looking for 4 wheel disc setups. I came across Baer brakes which are really the only ones that mentioned a master cylinder specs. Look down the page a bit and you'll get an idea what they think. I also have 17" wheels, so that may make a difference in more available options.  The setup listed isn't really on my radar, but who knows. I'm going to call them & a couple other places up, but by far, this place certainly puts more information out there.

https://baer.com/13-Front-SS4-Brake-System-4301526.html

This setup below says to use a 15/16" master.

https://baer.com/13-Classic-Series-Front-Brake-System-4301551.html

 

 

A clip of the chart they reference:

master.JPG

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7 hours ago, Ed Wieder said:

What difference, if any, would there be using a 1 inch bore master cylinder with disc/disc versus 1 1/8? Thanks, ejw71

Ed,

A 1" bore master cylinder will require less foot force (i.e. "softer" pedal) to generate the same brake line/wheel cylinder pressure than a 1 1/8" bore master cylinder.  Pressure (P) = Force (F) / Area (A) so if the Area (3.1416 x Radius x Radius) of the bore goes up, so does the Force (applied to the master cylinder piston) to create the same Pressure.  In this case, to generate a 100 psi line pressure, the 1" bore master cylinder would require 78.5 pounds of force while the 1 1/8" bore master cylinder would require 99.4 pounds of force or an increase of 26.5%.

Although, your brake pedal gives you a lot of mechanical leverage in creating the force on the piston, the 1" bore master cylinder will definitely have a "softer" feel when braking than any larger bore.  Of course, the larger bore sweeps more brake fluid volume so brake pedal travel will always be less than with a smaller diameter bore.  If you have power brakes, the booster takes care of the additional force required by a larger bore and gives you a shorter brake pedal travel - nice tradeoff.  If you don't have power brakes, the smaller master cylinder bore will require less foot force but a longer pedal travel given that the rest of the braking system remains the same.

BTW, the factory spec on a '70 Monte Carlo master cylinder is a 1 1/8" bore and I would imagine it was the same on the '71.

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