Brake Master Cyl Bore Size? - Impala Tech
Brakes & Suspension Conversion Questions & more

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post #1 of 7 (permalink) Old 04-21-2020, 07:28 AM Thread Starter
 
 
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Brake Master Cyl Bore Size?

Generally speaking, what creates more brake pressure from the master cylinder...a small bore size like a 1" or a larger bore size like 1.25" ?

i.e. the bigger the bore, the more pressure??? Or the other way around? Big less, small more pressure?
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post #2 of 7 (permalink) Old 04-21-2020, 12:22 PM
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It is all related by Pascal's Law: P = F / A. Where P is the brake line pressure and F is the force you apply at the pedal (in pounds if we are talking psi in pressure) and A is the are of the pistons in a closed system. Pascals law says the pressure is equal every where.

To do actual work such as move a brake component you have to vary the ratio of the master cylinder to wheel cylinder size. The bigger the wheel cylinder gets the less it will move with the same pressure. So you need to take into account the ratios, areas of the wheel cylinders with how far they will move to contact the drum or rotor to calculate the diameter of the master cylinder (which comes in about seven different diameters for most American cars).

Piece of cake once you have all the measurements accurately made it is just algebra. No calculus required. Can't figure out why engineerrs go crazy over these details.

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post #3 of 7 (permalink) Old 04-22-2020, 07:47 AM Thread Starter
 
 
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Thanks for the reply, but never took Algebra. Just need to know if pressure is increased or decreased with different size bores in the MC using front disk & rear drums.
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post #4 of 7 (permalink) Old 04-22-2020, 09:16 AM
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Quote:
Originally Posted by 1963er View Post
Thanks for the reply, but never took Algebra. Just need to know if pressure is increased or decreased with different size bores in the MC using front disk & rear drums.
Cant answer your question even though I am a degreed mechanical engineer, because it is the ratio, or fractional difference if you like fractions better than ratios, that determines force difference. It is the difference in size that determines what happens.

Example of what I am talking about.. You have a floor jack with a four foot long second class lever pushing on a hydraulic cylinder to pressurize it. Problem is the cylinder has a leak as all of the fluid you just pressurized goes into another cylinder that is ten times bigger in diameter and a lot longe. Ignoring the reed vales and spool vales that allow the fluid to move from the reservoir to the pumping cylinder on to the working cylinder; the thing that makes the jack work, is the ten times bigger part (it has probably a much bigger ratio, but ten is easy to multiply with).

As you pump the handle up and down you create pressure in the pumping cylinder (a lot of pressure) and that Force is multiplied by ten squared or 100 times the pressure. This then uses a mechanical linkage to lift a car weighing tons, but it requires a lot of strokes to do that. But you have asked only half the question as to the diameter of the master cylinder. You haven't mentioned the diameter of the wheel cylinders. Which is why I can not answer the question.



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post #5 of 7 (permalink) Old 04-22-2020, 12:01 PM
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If you ever want to make a short conversation longer, ask an engineer.


To sum up, smaller bore makes more pressure, but moves less fluid.


https://www.joesracing.com/master-cylinder-math/
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post #6 of 7 (permalink) Old 04-22-2020, 12:29 PM
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Quote:
To sum up, smaller bore makes more pressure, but moves less fluid.
You got half of it right. The volume of fluid moved is greater with a larger bore, but the pressure generated is the same regardless of bore diameter. That is a function of pedal pressure. Resistance or effort goes up with an increase in bore size which is why most manual brakes have a small bore and power assisted brakes have a larger bore size.

Brakes are complicated in terms of getting the parts to work with each other. use the wrong parts you not only waste money because they won't work properly but you could kill someone in your experimenting.

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post #7 of 7 (permalink) Old 04-23-2020, 08:08 AM Thread Starter
 
 
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Whew!!!
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