OK, I'll take a shot at this ...
A 5/8" cylinder has a surface area of roughly .31 inch sq.
A 3/4" cylinder has a surface area of roughly .44 inch sq.
Let's say for illustration the force applied by your foot multiplied by
the arm of the
brake pedal is 100lbs (I have no idea if this is even close, but it's
convenient).
Divide 100 by the respective areas and get:
On the 5/8" cyl the force applied to the brake fluid is roughly 323 psi.
The force applied on the 3/4" cyl is roughly 227 psi.
So, for the same pedal pressure, with the 5/8" m/c you get about 43%
more force at the
slave cylinders or, conversely, to get the same stopping power you'll
have to press the
pedal 43% harder. It's not the volume of fluid you push, but the force
applied to it that
matters (unless you have silicone brake fluid, which is apparently made
from old sponges
and is impossible for mortal man to bleed properly ;). The amount of
pedal travel variance
in the two would probably not even be noticeable (unless your slave
cylinders are very far
from the drums or disks, in which case you need to adjust your brakes).
This is why the servo-boosted systems have the larger bore
cylinders--the servo amplifies
the foot pressure and you need less of the hydraulic advantage provided
by the smaller bore
master cyls.
bs
Steve B. Gerow wrote:
>On my car the PO had fitted the .75 in. master cylinder which seems to work
>fine with my non-power drum brakes. The pedal effort is not up to modern
>standards but not excessive.
>
>If I were to switch to the 5/8 cylinder would my pedal effort get less or
>more?
>
>I'm thinking it would be less because I'd be moving the piston further to
>move the shoes the same distance.
>
>
>
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Bob Spidell San Jose, CA bspidell@comcast.net
'67 Austin-Healey 3000 '56 Austin-Healey 100M
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