[Tom, F&L GoR]

As an added comment, when I disassemble the old rear calipers sent in as cores for the 40mm resizing, it is obviously many do not flush their fluid. The needle bearing and load-bearing washer that support the parking brake arm are inside the caliper, bathed in brake fluid, and often rusty. Luckily they are are at the far end from the piston and piston seal, so I've only seen perhaps two or three where it would cause a fluid leak or stuck piston. While the rears may not get as hot as the fronts to cook the fluid, you should bleed them occasionally to dry the fluid.