Let's get this out of the way. It's best to bleed out the brake lines to get the old fluid out. It's the right way to do it.


There are some shops that just suck out the fluid out of the fluid reservoir. When the owner see it clean, they must have change it/replace it (therefore bleed it.). Meaning, (most) owners can't tell it if the system was bleed out or not. The reservoir is their best guess.


Some shops are dishonest, and some shops are just busy. And some technicians will just do that, cause they are lazy, or just busy. Shop manager gave them too much work. And said "it's done."


So my question is, if the system was not bleed out. And only replace new fluid in the reservoir, is there any chance of a fluid exchange between the old and new? Down to the lines? Or are the old fluid down at the brake line pretty much stuck there. There is only "one way" out for the old fluid.

 

When you replace pads you push the pistons back into the caliper and the fluid rises in the reservoir, so it can interchange. Not sure just how much that happens without a pad change.