>>1496789check if the cables are changing tension
Brifters have many small parts and need to be fairly clean and with fresh lubrication for the ratchet and pawl to work properly
its generally best to avoid dissembling them if at all possible
some types of grease dry out over time and become too thick
a little bit of penetrating oil can help otherwise you may have to clean out the old grease
https://www.bikeforums.net/bicycle-mechanics/886004-guide-repairing-shimano-rsx-a410-sti-levers.htmlre lubricate with something like white white lithium grease
>>1496791if the cables are old
some light lubrication of the cable bowdens can help
