I recall coming across a link about these, but can't find it again. When do these need replacing? I think the gum walls are cracking, but it's hard to tell if it's the tube pushing out at those cracks or just just the gum material swelling out a bit. The tread looks ok, again cracking in that area; but it seems to be holding air.
Also: I think these tubes are 20 years old. They held air over night, at 50psi. Prudent to replace, or just wait for a hole?