Fair warning, if you've never opened one up before, pay close attention to where things go and in what order they stack. I made that mistake when I first started using round reels and ended up having to take it to Turbofred to straighten it out. Also, be careful when you take out the spool that the little tubes on the spool brake don't fall off and disappear. Done that too and it's like trying to find a brown grain of sand on the beach.
Thankfully abu usually sends some extras in the box when you buy a new one. Ive don't that a few times as well.
Im betting on old grease gunking something up. Pay attention to how you take it apart get a paper towel or two and put the parts in order and proper direction as you take them off. Lightly grease what is greased and oil what is oiled and reassemble. Would be a good time to clean and grease your drag washers as well, will make it smooth as the day it was new again.