Originally Posted by NegaRutabaga
Unfortunately the max durability also goes down....just for repairing your equips.
Actually, you don't automatically lose max durability each time you repair your equipments. I believe it's around 50% chance of either lowering its max durability or restoring its durability(from 0/100 to 100/100).
For upgrading, the equipment's durability(not max dura) will only drop to 0 if you failed on your attempt.