Would you reuse this caliper piston?

[b.walker5]

And I'm not sure even the dishwasher would remove all the gunk I've found in those groves.

Most dishwasher detergents/powders are fairly caustic in nature, and combined with hot water and high pressure will remove most gunk. I've washed plenty of bike parts in mine, including engine cases and never had a poor result. It' great for cleaning things prior to painting but a tip, if you want to paint after washing dont use those power ball detergent things, or seperate rinse aid. The rinse aid is a wetting agent that promotes water run off on glassware (to leave it sparkling) but it leaves a residue behind that paint wont stick too. BTDT.