I get lots of calls from shops and a few owners who fully ignore our warnings to flush the brake fluid BEFORE you push the pistons in to install new pads. Across the board it is always the same, we never have to do that on other vehicles. While I do not know why that is the case, I do know that the TJ collects crap in the piston bore at a very prodigious rate. This is 3 year old or so fluid in a TJ that the owner takes reasonably good care of, it has reasonably low miles. Along with what you can see, there were also ridges of built up hardened crap at the end of the piston stuck to the side walls that would have trapped the piston hard and at least tried to ruin a set of new pads.