On some GM vehicles of the '80s, air pressure from the engine compartment allegedly kept the air box from draining. There's a right angle how to put on the drain tube to keep that from happening.
From what I know, the air box seal was a known problem, even when the cars were newer. Upon further investigation, the slide-on nuts should have been on the outside of the case, rather than only on one side, , meant the gasket (open cell rubber?) had to conform to those clips being there, sealing against them, with small gaps on each side of the clip. Worked for a while, back then. I was at the local dealership one day when a new customer had had the a/c case on her Fury III resealed twice already, with no improvement.
To me, the best way to do it would be to remove the case halves and put a bead of silicone on one side, letting it fully cure before assembly. Possibly even leaving the push-on nuts in place, or replacing them with a different style of retention, possibly external metal hex nuts instead? Key thing it to have good sealing on BOTH sides, with good retention.
Back then, I asked the local Chrysler (old-line) service manager what they changed. He said that they deleted the drain pan from the '69 design, which funneled the water to the drain, rather than letting it collect in the bottom of the case.
Back to the drain tube issue, they can get algae'd up with time, so a manual clean-out (as mentioned) might be a good first step. On some of the '80s GM fwd cars, they had the right angle drain, but with a collection area at the bottom, with a slit that stayed shut. When one of those cars was on the rack at a friend's service station, I noticed some wetness on the bottom of the hose, touched it, and a deluge followed. YIKES! About a quart of "water" came out, it seemed.
Just some thoughts and observations,
CBODY67
