You serviced the radiator as well? Sometimes, the leak can occur not only in the pipes but in the radiator unit. I encountered this problem where the reserve tank coolant drops very fast and I bring it to the radiator shop, once they look at it, they manage to determine that the radiator is leaking. For my case is the area where the upper portion of the panel where it is connected to the cap. It's looks like a "longkang" as the below highlighted area and that's the place where the coolant seeps out. After the repairman re-patch it, now it is good.

Pic:

not my car's pic