Originally Posted by elp_jc
I doubt you can feel ANY difference with just lines, when you're braking with your foot aided by a booster (vacuum assisted). I can BARELY feel the difference on a motorcycle, which has no booster and we're braking with our fingers. Of course the only exception is BMW bikes with servo brakes (like my K1200), where I wouldn't bother either.
But the other aspects, like degeneration over time, etc., are valid points. I'd only replace them if I tracked my car, but more for safety than feel. Good day.
I don't know much about the specifics of the brake circuit, so I'll think out loud here. The booster is on the input side of the master cylinder and drives the first master cylinder piston, which indirectly drives the second piston, and the two pistons force the fluid out into the brakes lines, right? So, once the two pistons travel enough so that the reservoir ports are sealed, you have whatever fluid you have in the brake lines at that point (sealed circuit). From then on the more pressure you apply on the pedal, the more pressure the booster will apply to the pistons, and the more pressure you will have in the sealed lines. However, since you have a closed volume on the other side, if the lines flex/expand and try to foce an increase in the volume of the sealed circuit, which can't happen since the fluid cannot expand, the master cylinder pistons will move further forward (since the caliper pistons are still under pressure and won't move back), and the pedal will drop lower. So the booster has nothing to do with keeping the volume of the closed circuit constant, and simply amplifies your pedal force to drive the initial master cylinder piston, and if the lines flex, your pedal will drop lower than it would if you had SS lines, which do not flex (as much). A somewhat similar issue with boiling brake fluid since that reduces the incompressible (fluid) volume and the master cylinder pistons will move further forward and the pedal will drop lower than they should while braking (once the reservoir ports are sealed).
That would be consistent with my experience after the upgrade: my pedal felt "firmer" in the sense that it did not seem to continue to drop while braking the way it with the rubber lines + ATE fluid.
Re: degeneration. Even with the SS lines, the lines are still rubber. It's just that the steel reinforcements would keep the rubber from flexing, so it might degenerate less? Also, if there was impact, the chances of the line being damaged/severed would be decreased?