Man, time to call in the plumber
Are you sure that there’s no internal faces that shouldn’t be there inside the pipe?
It looks as though you are using a fairly good resolution. Though I would still maybe try a higher resolution and also use thicker walls for the pipe (even if only temporarily before you bake the fluid simulation); or maybe even use a “single layer” mesh (be sure to set the obstacle to ‘Init Shell’ for an “open” mesh object ie. an object with no defined inside/outside).
Could you post the blend file? Would probably be faster to track down the issue that way.
I noticed one issue - it looks as though the horizontal section of the pipe (at the right side) has no faces at the end, meaning that you don’t have a closed section - that would cause issues if you have the obstacle set to ‘Init volume’.