727 2nd gear slipping

Adjust both bands! Kickdown band: Tighten adjuster to 72 inlbs. Back off 2 1/2 turns, hold and lock.
Low and reverse band: Tighten adjuster to 72 inlbs. Back off 2 turns, hold and lock. Check if your specs are slightly different.

Pull front trans cover. Rotate converter to check for a drain plug. If yes, drain the converter.
Change the trans filter. Verify that there only one o-ring. Clean the oil pan and magnet.

Add ATF+4. Check fluid level in neutral.