| First consider what’s the same:
• The volume of hydraulic fluid that it takes to move the pistons from their retracted position to where contact occurs between the brake pad and the rotor disc.
• The pressure in the hydraulic fluid for a given braking force between the pads and rotors.
• The stiffness of a given brake line construction, that is, it’s resistance to expansion both circumferentially and longitudinally.
Now consider what’s different:
• The total length of brake lines that runs between the master cylinder and the calipers. Two lines are longer than a split configuration (which is the key to the answer.)
When you pull the lever to the master cylinder, a small volume of fluid is displaced that moves the caliper pistons. Then the pressure inside the brake lines increases as you squeeze harder - more force is exerted by the master cylinder and the brake pads are in contact with the rotor.
As the pressure rises inside the brake lines they expand and elongate. The volume of fluid that they contain increases and some of the force you apply to the lever is being used to expand and stretch the brake lines. This effect gives the feel at the lever what’s called a “spongy” feel. Said another way, you loose part of the pull harder - brake harder effect needed to modulate the amount of braking.
The longer total length of dual brake lines will undergo a greater increase of internal volume (which is less desireable) than a split configuration.
What’s interesting is that brake line sizes are commonly either -3 (3mm I.D.) or -2 (2mm I.D.). 2mm lines will expand less circumferentially than 3mm lines. So if you run two lines, use the 2mm size to negate the spongy effect of the longer line length. Two 2mm lines will move almost the same amount of fluid as one 3mm line so flow restriction is not a concern.  |