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Not knowing what vehicle or year, except it is a Ford van and I assume in front of the radiator, I would say take the bottom line off, everything will flow out. If you feel a need to power flush it, then it will be flushed when you do the transmission.
Because I tow, drive off road and drive in the desert, I have the dealer flush my transmission through the lines every 30K. also a good time to change the air and fuel filters.
I am assuming you are rebuilding or replacing the transmission?
The best method to flush the cooler you can take the lines off the transmission and use a can of "cooler flush". This is a commercially available product that is an aerosol can with a nozel that fits the cooler lines. You then spray the can of flush through the lines and blow it out with compressed air. Works fine.
Or, you can slip a rubber hose over one of the lines long enough to reach up above the engine cover and poor some mineral spirits through it. Then blow that out with compressed air.
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