which car model
after using a turkey baster to siphon nearly all the old fluid out some dirt may remain along with some fluid
on some of the older Benz the brake reservoir pulls off (and can be rinsed and thoroughly dried) - though some techs don't like to pull the reservoir off. On my `98 C class the reservoir is bolted and awkward to remove unless the you start to pull the system apart. I had some tiny debris in the reservoir of my C and I just had to bleed not finding any way to wipe it clean.