Battery for Suzuki GN400T Bobber/ratbike......?

I bought a brand new batter for my 1981 suzuki gn400T rat bike/bobber, which i had to place the acid in myself, gave it to the mechanic to charge it the way the paper said. all light worked, only problem all light went dim very dim on my way home in the middle of the night. Does anyone have any clue why that would happen. I haven't changed any wiring to the bike, i checked the fuse good i followed all wiring nothing seemed to b wrong could it b the batter? I bought a Pro Start power sport battery, which im taking back. Wat type should i bye n i bought it at pep boys:(