| The pistons are retracted from the expansion of the seal rubber, as you brake the piston pushes against the seal, when you release lever the rubber expands pushing the piston back. As you can imagine the rubber only pushes it back a very small amount. Typically the rotor floats and gives a slight hiss as it passes pads but it also knocks the pistons back a tiny amount each revolution and the wheel then spins freely. This doesn't happen on a paddock stand, the only true test to see how much grab there is is to ride bike the bike and then try and stop and put it on a stand without touching the front brake lever, then spin wheel. As forks have got stiffer and tolerances on wheel bearings tighter there's less chance of the rotors moving and knocking pads back.
Theres been a couple of MTB systems which have a closed system which allows adustment of piston/pad clearance from master cyinder, sounds good but downside is you can build up enough heat in the system to expand fluid to a point where you can lock the wheel on long downhills dragging the brakes unless you use large rotors.  |