Master cylinder bores....what size?

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Tom, It ISN'T that simple. Seems to me you have basically three choices:

#1 You could just take a stab at it and well it'll be pretty much like throwing darts.....in the dark.

#2 You COULD take an "educated" guess by just copying a factory set up that you feel comfortable with, BUT!!! you better make sure you copy the ENTIRE system piece by piece.

#3 You could strap your geek hat on and do some simple calculations.

Things you'll NEED to consider - pedal ratio - in short it is the distance ratio as measured from the PERPENDICULAR line of action. What I mean by this is that let's say you have a pedal that's 12" long BUT you have it mounted at a 45 degree angle - in this case you do NOT get the FULL benefit of the 12" length - it is reduced in this case it is now EFFECTIVELY 8.5" - don't get hung up on that if it doesn't make sense - but think about it - and draw it out - it'll become crystal clear.
Remember the lever effect will occur on BOTH sides of your equation - at the pedal and at the clutch fork.
Now jump over to your bore diameters. Let's give an example - a 1-1/8" bore has an area of 1 sq in. A = pi * R * R or .562 (squared) = .31640 multiply that by pi (3.1415) = .9939 sq in - or basically 1 sq inch. So now that we got that...... if you push on your clutch pedal with a force of 25# and you have a pedal ratio of 4:1 - the force is now multiplied to become 100# - now put that force "into" your 1-1/8" bore master cylinder (clutch cylinder) and you get a line pressure of 100 psi ----->force (in lbs) divide by area (sq in) = PSI (pounds per square inch) so that would have been our 100# force DIVIDED by our 1 sq in AREA to result in our 100 PSI. now REVERSE this on the opposite side. Say you have a 3/4" bore slave cylinder (area of .44177 sq in) - so back to the PSI equation: PSI= Force / (area) . So we take our 100PSI = Force ( .44177) so basically we solve fo "X" (which is FORCE in this case) so.... 100PSI multiply it by your area to get the OUTPUT FORCE of your slave cylinder - in this example: PSI = 100, area = .44177 so 100 (.44177)= 44LBS POUNDS of force at the slave output shaft - multiply this AGAIN by your clutch fork ratio and lets say it 3:1 and you get a force of 132 pounds. or basically LESS than what you started. SO if you want to boil it all down - MORE INPUT AREA makes it HARDER to push the pedal, Less Input area makes it EASIER to push the pedal but costs you more pedal travel. Sizing your M/C (clutch cylinder) SMALLER than your SLAVE CYLINDER gives you INCREASED leverage at the cost of travel.

Bottom line EVERYTHING is a teeter totter - (simple leverage) - you use MORE movement with less force to get HIGHER forces with a little movement. Brakes /hydraulic clutches work exactly the same way. You use a larger bore master cylinder and you'll get more volume with increased pedal effort and LESS pedal travel. Use a smaller bore diameter and you'll get reduced pedal effort - more tavel and potentially less volume. It's ALL just a balancing act!

Hot rodders way - pick the one with the LESSER diameter and see if you can stand the travel - if not go the other way

Keep this in mind - if EVERYTHING was OK to begin with and you simply wanted to go "wet" keeping the bore diameters the same will effectively eliminate them from your calculations and just do the pedal ratio's - if the pedal ratio's don't get you where you need to be then you can fiddle with the bore diameters.

The biggest thing is to make sure you MULTIPLY or DIVIDE properly - sketch out what you are doing and make sure it all makes sense. For instance is the clutch fork ratio 3:1 or 1:3 - BIG DIFFERENCE!!!!!

In short !!! the PRESSURE is the same (its a sealed system) so the ratios HAVE to equal out!! A SMALL bore clutch cylinder and a LARGE bore slave cylinder giives you MORE FORCE at the clutch (or less operator force required)

Remember if there's MORE area - there's more FORCE since it IS dependent on the area - 1PSI ain't a whole lot of force but if it's acting on a area of 1,000 square inches then it's a force of 1,000# pushing on it!!!! Remember 1 psi is 1 pound PER square inch - more square inches equals more force!!

Just thinking out loud......

 

Are you talking about a clutch slave cylinder or are you calling wheel brake cylinders slave cylinders (which they are but, the issue is confused)?
If you're talking about clutch slave cylinder, it would depend on if you have a "regular" or "heavy duty" higher pressure clutch in it.
Higher pressure pressure plate you would want the bigger slave cylinder.

 

No.
They are paired up according to required pressure ratio requirements.
You're building a hotrod?
You get to experiment with what works best for your car!

Cylinder ratios work similar to gear ratios, or (opposite) lever length ratios.
If you use the '40 Ford front brakes on a car with the same front/rear weight bias as a forty Ford (Yea, they were all different by model werent they?)
you need to use the same SIZE rear cylinders IF the rear brakes are the same diameter and shoe width as a forty Ford.
Confusing enough yet?
If the ratio between the M/C and wheel cylinders is different from the 40 Ford, you can compensate by changing the pedal lever ratio, or with a heavier or lighter braking foot...
I'd think the truck was similar in weight to the 40 Ford so the brake parts were probably also wthin a useable/functional range.
The 40 Ford brakes are only about half as functional as a self energizing brake so lean towards a smaller rear cylinder if anything.

 

Think of it this way:
Larger bore master = less travel to actuate, less pressure.
Smaller bore master = more travel, more pressure.
By pressure, I mean the multiplication of effort.

Since it seems that you are limited in master choice by choosing to use a paired unit, you will have to 'tune' the system with slave and wheel cylinder sizing.

Be prepared to spend some money getting this right, or please don't drive near me!!!

A really good book on brakes is available from Amazon, click here.

Make sure you have a proper brake system, FIRST.
Four drums require naught more than a master and wheel cylinders, lucky you.
Once you are set, drive the car and panic stop it.
Which locked up first??
Front?? Go to a larger bore in the REAR.
Rear?? Smaller bore in the rear.
I tell you rear, because 40 fronts won't allow such tuning, I think.
Neither?? Larger bore all 'round (or smaller master bore, but your system won't allow that).
Modern rear brakes, in most cases, will have a wide variety of bore sizes available to you, but you'll have to hunt a little, don't choose just by application. NAPA have a picture book which is wonderful for this.
After, and ONLY after you have tuned the system this way, should you even consider an adjustable proportioning valve. These are for fine tuning ONLY, though misused daily nevertheless.

Hope this is clear,

Cosmo