AC condensate defies gravity

This was back in 2014.

Another well known plumbing company recently replaced filters in my customers AC unit located in the ceiling of a closet.
After they left the drain vent would burp out water every time someone use the sink that the drain was connected to.
They came back twice with their best guys and could not figure out why.
Water does not flow up hill on it's own so why was water coming out of vent on condensate drain that was at least 6 ft higher than the connection to the sink drain.

PIC #1 AC condensate drain in closet ceiling.
PIC #2 Existing AC drain connection to sink.
PIC #3 My repair to the drain connection at sink.

The Cause of leakage from Condensate vent.

The drain connection ( hose to branch tail pc ) in PIC#2
same level as drain into wall.
When someone turned on the sink the drain was partially blocked and the AIR trapped in the condensate line below the water flow was burping the water in the PVC trap on the condensate line.
When the other guys replaced the filter, they bumped the drain and back graded it a little.

So I re-graded the drain. replaced the pop-up assembly with a quality brass one with a threaded tail pc

I love it when I solve a problem that one of the biggest Plumbing companies in town could not figure out.