ISTR old batteries will have more heat loss when charging and when you get down to the end for tapering amps, the lowest the amps get to will be higher, where more of those amps are going to heat instead of charging.
So your "end current" or whatever you call it, will be higher than with new batteries.
LY has seen with AGMs (me too) that after they get down to "end current" at 0.5a/100AH and you leave them at 14.x volts they will keep warming up and that takes more amps, so amps start going back up. Time to lower the voltage!
I am guessing these 8v batts are for a golf car. You can use four T-1275 12v or six 8v to get your 48v.
What is the make/model of your charger if it is not one of those kind that comes with a golf car? It has settings of interest not found in many chargers.
