Flushing of A/C lines, radiators, transmission coolers, and power steering systems are recognized practices to be performed after catastrophic failures or extreme corrosion when encountered in radiators. This practice is NOT required or recommended for normal service operations.
If you feel a need to do it before the recommended 5 year/100,000 mile interval, simply drain and replace. This is especially true with the transmission. Flushing it could knock some dirt loose and cause a catastrophic transmission failure.
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