All 100 & 200 series power steering reservoirs I work on, are that clean

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I start by removing & cleaning the reservoir in a sonic-cleaner w/de-greaser. Removing old gunk settlement in bottom and cleans the internal screen.
Cleaning power steering reservoir and flushing it's fluid. Keeps Vane pump and rack & pinion in top condition. These step will help you get, a better idea on how to. Power wash reservoir & hoses using a degreaser, before starting service. Makes for cleaner job. Repeat, once all done with...
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BTW: Toyota ATF is made by Exon Mobil. Toyota & Lexus Dealerships around here, used Mobil One ATF. When they've a transmission that calls for Dexron II or III, that's running hot. Otherwise they use bulk Dexron III (98-02 100 series). I use Mobil One ATF for all 98-02, full 12 qt flushes. Or the Exxon Mobil one shelve down; Mobil MV ATF full synthetic, for all 98-up. It's (Mobil MV FS ATF) new and rated for Dexon II & III, Toyota IV and WS. I'd use Mobil One in all. But Mobil One has NOT rated it for: Toyota IV or WS, in North American market.
Mobil One is top shelve and readily available thorough North America.
Mobil MV AFT FS, is still hard to get.