My previous 2 cars have had 4 pot Brembo's on the front & both have suffered from this. The calipers are aluminium & the sliders in which the pads move along are stainless steel. Corrosion builds up under the sliders & in time they lift slightly, effectively reducing the gap in which the new pads fit.   Yesterday I set out to replace the front pads. Gladly all pistons & boots were moving freely, old pads were Delphi, not sure who makes the grooved discs though. I starte