When you remove the return trans fluid cooler line for a complete power drain you might be able to use the two ring seals.. BUT
To to prevent potential fluid weeping on the end of this pipe you might want to replace them if you end of removing the pipe in your procedure.
722.9 Transmission Oil Cooler Line to Transmission Housing Seals x 2
$8.40
SKU:
TRS-722.9COOLERSEALS 
Product Details
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722.9 Transmission Fluid Fill and Power Drain Service Kit
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Early 722.9 Transmission Pan and Filter Upgrade Kit w/ Video Instructions
To increase fluid flow. The best thing you can do to increase the life of your 2003 to mid-2010 7-speed transmission.
Many enthusiasts are now buying these 2004 to mid-2010 high-performance Mercedes with little thought to the transmission.