Originally Posted by
oldbobcat
Right.
Bicycle hydraulic brakes are self-adjusting. That's why when you squeeze the lever without a rotor between the pads you have to pry the caliper open to get the wheel back on. If you doubt this, take a look at your caliper with worn pads and tell me if the gap between the pads and the rotor has increased.
A soft brake lever means there's air in the brake line. By all means, check your pads for wear, but when your lever bottoms out it's telling you it's time for a bleed.
You're confusing self adjust piston position with compensating for lever throw. No one is saying the pistons don't compensate for wear. Also, an increasing lever throw doesn't mean the braking is soft. Just that there is effectively more free stroke before the brakes engage. I'm only familiar with Shimano hydraulic brakes. With these there is a small "reservoir" that has a diaphragm that more or less allows the system to be at ambient air pressure when not engaged. As the pistons move out due to pad and rotor wear, the extra fluid needed comes from the reservoir. This, I believe (but do not know) is to help compensate for the gradual movement of the pistons due to wear. But, it's a sealed system. So, the system must gradually develop a slightly negative pressure relative to ambient, as the diaphragm expands into the reservoir. I believe it's this reason that the compensation is not perfect.
I believe that without the reservoir and diaphragm system, the change in throw would be extreme, but again the system is not prefect. So, there is an effect on lever throw as the pads wear.