What i use is a *second* tee at the lowest point of the loop, with a simple cap. When i want to purge the system (and remove tubing..) i open the airtrap (at top) then the cap at bottom, and all the coolant flushes into a basin i put there (do *not* forget the basin
).
When unplugging tubes, i stuff paper towels into openings to prevent remaining drops from going anywhere.