CH, thanks for the pics - they make it easier to see what you're dealing with. The channels all look good in the pic. The gasket itself appears solid, but in pic #2 it looks like some warpage on the right side of the large section (this is the part that protects the waste line when in normal filter position) - did you look at the sealing plate (the part that is in contact with the gasket) for irregularities? Something caused that warp on the gasket and it's in the right spot to cause the problem we've discussed. I've got a possible fix in mind, but want to hear back from you before I suggest it.
Slow and steady... we'll get to the bottom of this before your water empties to the bottom of the pool - Waste