If it's old, tear it down.
No, I don't mean all the nice houses in my neighborhood, though some greedy bastards are trying.
I mean you have to take your forks apart to clean the innards. I don't reckon flushing would be effective. Serious spooge to wipe outta there.
Overhauling forks is not that big a deal, really, just make sure you have new seals and washers and such, and a tool to keep the damper rod from turning. Don't use solvents on the rubber/plastic bits. Don't scratch, bend, or crush anything. You know, the usual warnings, only more like brakes than motors. Oh, and do one leg at a time so you know how it's supposed to go back together.
Let's see dgrs47 originally asked about the forks in a 01 640 R...those are old-school right-side-up forks, right, WP ? Shoot, drain and fill and go ride. No worries.