Please be advised that special service tools are necessary to disassemble and service a rack assembly. When they were first introduced, they were serviced on the bench but now are R&R'd for the most part.
What has happened is the seals on the spool valve have hardened and started to rub into the aluminum valve bore. That is the gray you see in the fluid. There is no fix for a damaged bore other than housing replacement or a rebuilt unit. A quality rebuild (Moog) will have stainless steel liners at the spool valve and both of the rack seals.
Service replacement calls for full system flush and an inline filter to be installed in the return line. The rebuilt replacement (long rack) will also come with innner ball sockets and boots, heck of a deal.
It's just like halfshafts...it is cheaper to install an exchange than it is to disassemble, clean and inspect, lube and reassemble.
I couldn't imagine driving a car nowadays without a rack. The old linkage technolgy is archaic. I have a son-in-law that has an '86 Camaro with a box and linkage. Too slow! Too expensive to replace all those service parts when worn.