Originally Posted by
keifer
This keeps happening, on both rims: the tube develops bulges on either side of the stem along the seam where the reenforcing rubber mets the regular tube body. One of these bulges then tears.
The clue is in the quoted text. The valve section of the tube is getting pinched between the rim and the tire beads.
After seating the second tire bead in the rim, you need to push the valve stem into the tire before inflating. This will get rid of the bulge and seat the valve section of the tube inside the tire.