Submitted by Skip: (pococj)

Once again, this stuff comes from a bunch of different sources. I stole it myself, saved it to a file, then cobbed it all together so even a buckethead like me could make some sense of it.

I

Master Cylinder to Wheel Cylinder Ratio

The overall ratio determines brake "feel". There is a "sweet spot" in the range of ratios. A ratio in the 27:1 range gives 2 finger power brakes, and feeling some line and/or caliper flex. A 23:1 ratio is at the other end of the spectrum; firm. Ratios lower than 20:1 can result in a feel extremely stiff and give a toggle switch effect; nothing seems to happen until the wheel locks. (By the way, this ain't good!)

Disc and wheel diameters must be taken into consideration, also. A 10 inch disc working against a 19 inch wheel doesn't have the leverage that a 13 inch disc working a 17 inch wheel. And the hand or foot lever ratio counts, too.

Of course for most of us dummies a quick and dirty “formula� is needed. After all, how many of us can figure out these ratios? Heck, I don’t even know what the basic formula is, let alone what figures to plug in! So, on to Part II.

II

Bore Sizes

2 pot single caliper: 9/16 bore.
2 pot dual calipers: 9/16 or 5/8 bore.
4 pot single caliper: 9/16 or 5/8 bore.
4 pot dual caliper: 11/16 or 3/4 bore.
6 pot single caliper: 9/16 or 5/8 bore
6 pot dual caliper: 11/16 or 3/4 bore.

A smaller bore master cylinder will increase line pressure and therefore braking power, but it also may not move enough fluid to completely activate the piston(s) in your caliper(s). The feel of the brake system is greatly affected by bore size. A too small master cylinder will give a stiff feel; too large a bore, a mushy feel.