Help deciding on MC bore size

[RET_COP]

I'm buying a new Master Cylinder and couldn't decide between a 3/4, 7/8 or 1" bore
My calipers are single pistons 87 Lincoln Mark VII for rear disc and 1978-1984 Chevrolet Monte Carlo single piston fronts disc I have manual brakes. I currently have a 1 1/8 and the pedal is hard to push for my liking. I'm staying with my current setup and just want more line pressure. Any suggestions?
Thanks

[jmimac351]

Read thru Stoptech's website. They offer a lot of technical detail about setting up braking systems - especially as it relates to MC sizing. You need to look at your MC bore sizes, caliper piston sizes, and measure your pedal to get the ratio. They have formulas there for zeroing in on it. I have not used these formulas to experience their effect, but I plan to with my current project.

StopTech : Balanced Brake Upgrades

"1) Line pressure can only be increased by either increasing the mechanical pedal ratio or by decreasing the master cylinder diameter. In either case the pedal travel will be increased."

[Tom Kirkham]

Assuming nothing else changes then,

Diameter^2/Diameter^2=pressure/pressure=travel/travel

1.125x1.125/(1x1)=1.266 or 27% increase in line pressure and pedal travel
1.125x1.125/(7/8*7/8)=1.65 or 65% increase...
1.125x1.125/(.75*.75)=2.25 or a 1.25% increase...

Hope this helps

[jmimac351]

It's handy having the Kirkhams on retainer.

[Barnsnake]

With 1.25" cylinders I'm sure your required pedal pressure is REALLY high.

Assuming your pedals have the same lever ratio that I'm familiar with:
Many people with that brake combo use a 7/8" rear and a 3/4" front. It's a reasonable compromise of effort versus travel. Another approach is a 3/4" rear and a 5/8" front. Lots of braking power, but considerable pedal travel. The .700" front makes a good compromise between the two.

See what the brake manufacturer's on-line calculators say. In my experience they tend to lean toward the high-effort, short-travel setups.

Don't forget that the coefficient of friction of your pads will also change the pedal effort with the same mechanical advantage.

If you've been driving it with 1.25" cylinders, you'll be thrilled with any of the smaller options.