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>if the pedal drops due to piston sucking, then, why is there never air in the system again?
I didn't say it right, when describing what I observed before. It looks like it is getting "sucked" back, but there was never anything done to allow air back in. By saying that, I am admitting a poor practice, in that my pad replacement sometimes has me pushing the pistons back flush without opening bleeder valves, allowing the possibility of pushing crud back upstream. The springiness that translates into a spongy pedal I'm attributing to the flexing of the piston's seal and the pile of (2) new shims which have yet to conform to the piston and pad surfaces.
Somehow I figured I'd pull it apart the next weekend and see which was the bigger or real factor, but by then the pedal travel was less or I got used to it. My Volvo-love-at-first-brake-job experience was how quickly pads could be swapped.
I've also seen what Bob reports where the rotor actually moves one way or the other, but then bearing adjustment or sticking piston or pad ways (glide) should all be checked I suppose.
>the shims are for brake squeal prevention
I fought a squeal for weeks in our first Volvo - rear brakes sounded like a school bus without regard for which type of shims, lube or goo. Breaking the glaze on the rotor would stop it for a week, but the final discovery was properly positioning that ATE notch in the piston -- the 20 degree thing. But nobody has ever told me that worked for them, except the volvo brake manual. That disk would just sing out to the world.
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