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Unlike mechanical (cable) disc brakes which feature a brake pad spring and where you have to adjust pad distance manually; hydraulic disc brake pad distance is regulated automatically and many hydros have no piston retraction spring at all.
Does the lack of pad distance adjustment make hydraulic disc brakes more susceptible to rubbing on slightly warped rotors?
This is a little bit of a two-layered question.
The literal answer is yes. Given no gap adjustability, if your priority was to avoid rub, a hydraulic brake would be more susceptible to having rub, or more difficult to set up without it, for any given amount of rotor misalignment.
What makes it more complex is that you usually also have the priority of having it be a good brake. Hydraulic brakes transfer power from the lever more efficiently, so for a given amount of braking power output they can have a wider gap. (In terms of their design versus a mechanical.) This is a subtle point because we're not talking about a much bigger gap, maybe not even appreciably, but it's true that it being hydraulic at least moves the needle in that direction.
Mechanical brakes with their pad adjusters and barrel adjusters don't really free you from having to keep the rotor just about as true as true as with a hydro. With most of them, if you back out the adjusters much at all to compensate for rotor warp, the lever is going to be hitting the bar and you won't have much of a brake. It is true that in some cases you are able to adjust the brake in response to slight misalignment and still have it work, albeit with that much more imminence of the lever bottoming.
Different brakes, both cable and hydraulic, also have some variance in how much leverage they employ. The more leverage, the narrower the gap but the more power is generated. The less leverage, the wider of a gap (rotor truth tolerance) you have, but less hard the rotor winds up getting squeezed. There are also various systems that change the leverage (aka mechanical advantage) of the system at different points of the brake lever stroke and/or armature travel.
There are some mechanical disc calipers that are very gappy and tolerant of poor rotor truth, but they're not good brakes. I believe the department store disc brakes are tending to play this trick these days.
@RoboKaren, answering your question with my personal experience, and may be better as a comment, but I don't have sufficient reputation points to comment; I have hydraulic Shimano M355 disks, and the rear rotor has the smallest of warps. My LBS and myself have been able to set the brakes up such that the warp does not interfere / rub on the pads, when the bike is in the workshop. However, after a couple of hard pulls "on the road", rubbing returns, as the auto-adjustment of the pads takes place. Between us, we have changed pads, bled / re-bled the system, adjusted brake lever, but the rub always returns. So, from my experience, I would say, yes, the lack of pad adjustment does make my hydraulic brake system more susceptible to rotor rub, although whether this is true for all other systems I cannot say.
