How can I remove a specific part of a brep?

check this post:

first of all it is not a grasshopper problem but you are facing:

  • a self intersecting surface
  • a far from orgin issue

don t expect that other s work through a defintion like this:

Dependend on several plug-ins.

first step:
bake the geometry and find the rhino-modelling workflow, to achieve what you want.
2nd
then transfer this manual workflow to gh.