The cleanest way to specify a sector filter is to use the '.element_of' method. An equality filter can be made by using the '.eq' method. In your case, something like this might be what you want.
pipe.set_screen(base_universe & POR.eq(0.0) & sector.element_of([239, 77]))
Note, it' typically not a good idea to do equality comparisons with real numbers (eg factor results) since there can be rounding differences. Maybe use two inequalities similar to what you had?
pipe.set_screen(base_universe & (POR < 0.01 & POR > -.01) & sector.element_of([239, 77]))
Just a thought. Good luck.