TOPIC:

You may want to look close at the cutout in the tank for the pickup tube.  If the cutout is small enough, it will physically hold the tube in place and keep it from falling off inside the tank.

Nessism, you hit that right on the head. Just rebuilt the petcock and when test fitting it I noticed exactly that. With the tank upside down, the tube simple rests on a lip and the peacock assembly screws right down over it. I didn’t catch that when I disassembled it. All cleaned up, rebuilt, and together now. 

Thank again to everyone who kicked in ideas