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dyermullet

Stock master cylinder for 71 monte 1" or 1 1/8"

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The AMA Specifications manual for a 1970 Monte Carlo says the bore diameter of the master cylinder is 1.125" (1 1/8").  I doubt that it was changed for '71.  

 

As you probably know, the Delco-Moraine and Bendix master cylinders were used interchangeably in the assembly plants when our Montes were built.  While the pistons in each unit are different lengths and configurations, they are the same diameter at 1 1/8". 

 

I replaced the Delco-Moraine unit on my '70 SS with a rebuilt Bendix unit which has since developed a small leak.  I really prefer the looks of the Delco-Moraine (on left in photo below) master cylinder so I am going to rebuild it and swap it for the Bendix unit currently installed.

 

post-567-0-34884100-1487961865_thumb.jpg

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Thank you for confirming the correct diameter.  I had already went ahead and ordered a 1 1/8" since the car was power brakes I made the assumption that it would give me the better performance.  I didn't want to have the longer pedal travel required of the 1" piston. 

 

New (not reman) MC was $2 more than a rebuild kit, so I am going to try that route first before the hassle of rebuilding mine.

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You are correct that a 1" piston diameter will require a longer pedal stroke to move the same volume of brake fluid to the wheel cylinders.  It seems intuitive that a larger diameter master cylinder piston will give better performance (i.e. more braking power) but the opposite is actually the case.  Since the master cylinder generates the line pressure received by the wheel cylinders, a smaller piston area (pi X radius squared) will give higher line pressure than a larger piston area for any given input force (i.e. pounds of foot force to the brake pedal).

 

Better "braking force" at the wheels can be achieved either by larger diameter wheel cylinders (since they convert line pressure to force on the brake shoe or caliper) or a smaller diameter master cylinder.  There are trade-offs in length of pedal travel with either option.  Best to just stick with the stock configuration unless you have some really unusual application.  AMA specs for the stock brake system say that every 100 lbs of force applied to the brake pedal generates 805 psi of line pressure - no wonder brake lines are made of steel tubing!

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