After riding only 820 miles on my ebike, I heard and discovered metal-to-metal contact on the rear hydraulic disc brake pad-to-caliper. One of the pads was worn into a wedge shape, rather than being worn evenly from top to bottom. The piston on that side would not retract and I couldn't install new pads, so I tore the caliper apart. Getting the piston out took considerable effort and I discovered that the O-ring was essentially glued to the piston on roughly 40% of its circumference. I had to scrape the O-ring off the piston with a knife. First question: how could this have happened? Did the caliper overheat and have the O-ring melt onto the piston? By the way, there is no groove in the piston for the O-ring. The groove is in the caliper body. Second question: is there a source of O-rings that measure in millimeters: OD = 24.5 mm, ID = 20.3 mm, width = 2.15 mm, wall height = 2.1 mm ? The cross-section of the O-ring is square, not round. The ebike company offered to sell me a new caliper, but I only need an O-ring. This caliper is very easy to rebuild, if I can just find an O-ring. I have done a thorough online search for O-rings and came up empty for this size.