Let's define some names:See the color-coded diagram in post 32 and its red arrows indicating flow direction when the cap is removed. I'm pretty sure the lowest chamber is connected to the main unfiltered oil chamber.
C: clean filtered oil above the upper O-ring
D: the middle section between the 2 O-rings
L: the low section below the lower O-ring
1) why the drain 11 of the D section is higher than unfiltered oil trapped in the lower L section? I means they could have the drain hole at the lowest section L, and the supply unfiltered oil enters section D, and this makes more sense for most of the oil to be drained off with the drain hole at the lowest section.
2) If the drain hole connected to the D section, then why the center rod has the hole there for what purpose?