Dear Won,
If the Oring is in the thread, you can replace the O-ring with BP-WTP-O10-BK.
https://shop.bitspower.com/index.php?route=product/product&path=59_128_374&product_id=404
But if the O-ring is inside the rotary, it could not be opened and replaced the O-ring.
Also, we advise you to only use our coolant BPTA-CMCD500 as the coolant.
https://shop.bitspower.com/index.php?route=product/product&product_id=7443&search=500m