Sorry for the error - I don't have a GMC, never saw a reference to the pick-up, even the pump pictures don't show one, but the literature does refer to a "floating inlet screen".
However, the picture is confusing.
It looks like the pickup is designed to pivot inside the cover plate, but what's that cotter pin doing? if it secures something, the part it's holding is the outer sleeve for the pick-up tube, but what seals them?
The only way that's going to be air-tight without a seal is if the pump cover itself is always submerged.
The original purpose may have been as simple as allowing the pick-up head to track small changes in oil level due to long service interval or minor leak.
This explains why hard braking will break the siphon - the pick-up head can't move to follow the oil volume since it's travel is pretty limited, and even if it did once the tube entry into the pump cover is exposed you have an air leak.
