I took the inspection cap off of my batteries because they have been weak as of late. Each were really low. I was wondering how effective it would be just to fill them back up with distilled water or is their a certain point at which you just have to break down and buy some new ones?

 

Refill with water then put a slow rate charger on they should be OK then...if you can take them to be load tested after you have them fully charged

 

What pappy said. Use good clean preferably distilled water and do a slow charge. Be careful adding water and fill only to the correct level. Then take and have them load tested individually (disconnect the heavy wire between the two very carefully also). May save you some money just outright buying new ones. Probably what you will find is that one is weaker than the other especially if they are three or more years old. Both trucks I have had that used the dual battery set up, the right battery went bad first and drug the left battery down with it. You probably can replace just one but I do both of them when one goes bad.<br>Good luck and be careful with these batteries especially when charging and disconnecting or reconnecting.