30-10-19, 12:10 PM
I’m guessing the washers you refer to are just to align the centre of the caliper with the disc. One piston needing to be a millimetre further out than the other to compensate for any slight misalignment shouldn’t be much of an issue.
Most likely it’s the seals as they provide the restoring force to pull the piston back when you lift off the brake lever. If they’re old or non-oem the piston will bind and roast the disc.