Originally Posted by AP - Chris_B

Pressure isn't lost with rubber lines, but fluid volume is. In other words, as I press the pedal and the rubber lines expand, more brake fluid must be provided by the master cylinder to keep filling the growing line volume. With SS lines, nearly zero growth means less pedal travel at the same pressure level as compared to OE lines.

OE rubber lines are the main pathway for moisture ingress in the brake system. This is why most manufacturers recommend changing brake fluid every two years. It's the moisture that lowers the boiling point and can eventually cause internal component corrosion. SS lines do not allow moisture through them, keeping the fluid "drier" -- or with a lower moisture content for a longer period of time.