The size of a Master Cylinder is inversely proportional to the force it can exert. In other words, a larger diameter requires a higher force on the brake pedal to generate a certain hydraulic pressure than a smaller diameter, everything else being equal.

The trade-off is that the smaller diameter requires a longer stroke of the brake pedal to displace the same amount of fluid.

When choosing a master cylinder, find the available size that is closest to the calculated size. When the calculated size ends up roughly in between two available sizes, go with the larger size. This is the safer choice, since it is always better to NOT run out of stroke on your brake pedal, than to have to push a little harder than is comfortable, in order to achieve maximum possible deceleration.

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In case you are working with a tandem M/C, the calculated bore sizes refer to an imaginary situation in which the front and rear hydraulic circuits are independently pressurized.

However, in a tandem M/C, hydraulic pressure behind the primary piston and secondary piston are the same, regardless any differences in diameter between both pistons (because the secondary M/C piston functions as a slave cylinder piston).

Car manufacturers get the bias right by careful selection of all other components that affect bias, and with help of a proportioning valve. Any difference between front and rear bore size (stepped master cylinders are rare but not unheard of) is meant to deal with differences in fluid volume requirements.

So if you want to make changes to your brake system and are working with a tandem M/C, you could do the following:

• Try to find components that result in (nearly) identical M/C bores front to rear, such as: effective rotor radius, total caliper piston area, friction of brake pads, or even: changes in static weight distribution, CG-height and rolling radius of tires.


• If the calculated rear M/C bore is larger than the front M/C bore, and you don't want to adjust (any further) with different components, use an adjustable proportioning valve to reduce pressure in the rear circuit. Remember, the rear (slave) M/C piston is simply replicating the pressure behind the front (master) M/C piston, and the larger calculated bore size for the rear MC is simply indicating that the rear brakes are too powerfull and will lock up way before the front brakes will, if no further action is taken.

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For single master cylinders (to install in a pedal box with balance bar), the available (current) sizes are:

		0.551"	≈	14.0mm
		0.591	≈	15.0mm
5/8"	=	0.625"	≈	15.9mm
		0.661	≈	16.8mm
11/16"	=	0.6875"	≈	17.5mm
		0.700"	≈	17.8mm
3/4"	=	0.750"	≈	19.1mm
13/16"	=	0.8125"	≈	20.6mm
7/8"	=	0.875"	≈	22.2mm
15/16"	=	0.9375"	≈	23.8mm
1"	=	1.000"	≈	25.4mm
1-1/32"	=	1.031"	≈	26.2mm
1/1/16"	=	1.0625"	≈	27.0mm
1-3/32"	=	1.094"	≈	27.8mm
1-1/8"	=	1.125"	≈	28.6mm
1-5/32"	=	1.156"	≈	29.4mm
1-3/16"	=	1.1875"	≈	30.2mm
1-1/4"	=	1.250"	≈	31.8mm

Single master cylinders are usually available in two stroke lengths: 1" and 1.25" (go with 1.25" if possible).

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Tandem aftermarket master cylinders come in the following sizes:

7/8"	=	0.875"	≈	22.2mm
15/16"	=	0.9375"	≈	23.8mm
1"	=	1.000"	≈	25.4mm
1-1/32"	=	1.031"	≈	26.2mm
1/1/16"	=	1.0625"	≈	27.0mm
1-3/32"	=	1.094"	≈	27.8mm
1-1/8"	=	1.125"	≈	28.6mm

Bigger sizes are available from the auto parts store for 2-ton trucks and up. And smaller sizes are available as OE parts for small European or Japanese cars.