Try getting some new ATF+4 (which is synthetic) into system.
At wallmart, in the jerry cans isle, there is a good suction device $4 called a flo tool-mix mizer walmart # 1042459
I use this to suction out old fluid & replace with fresh ATF+4 occasionally.
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I didn't want to drive to Wal-Mart, and then I remembered that I live on a horse farm and have tons of syringes in our sharps box! So I got a 12cc and went to town. The fluid that came out wasn't exactly red, but more yellowish:
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I hit a point where there was a baffle, and I couldn't go any further, so I attached a plastic straw(I'm hoarding them as some NJ towns are outlawing them) and it allowed me to get to the bottom of the tank via the back right corner:
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I sucked out this much fluid and you can see some fresh ATF+4 on the towel next to the stuff I sucked out and the color difference. The tanks now filled and very red! And yes, I'm gonna hit that tank with some cleaner+a hose when I get to the store next.
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We'll see if when the temp dives in a few days if it screams.