As above, you should be checking amps between post and cable, not volts. Better to use 10A tester just in case. If you r old battery couldn't produce more than 10.5V after a good charging, I'd say that would be a suspect. How old is /was it? I'd be focusing on current draw from new pump with ignition off. Checking wires for voltage is useful if you're looking a drop in current, which you're not. A short will produce fire works and pop the fuse. I'd do an amp test on new battery before connecting the cable. If minimal amps show, off you go. Otherwise, do fuse test.There are as I recall some non-fused circuits, and I think a bad starter can cause battery drain. But from your description, I'd say the battery chose that moment to expire.