whith old chimney's they dont have lead flashings in them . you probably have a cement fillit at the base of the stack ,, which is probably all cracked and thats why its letting water in . you could knock the fillet out and cement a new one in that would stop the leak for a while , if you want lead in there get a roofer to do it , its a tricky job and needs to be done by someone who knows what there doing
RICH THE ROOFER
October 2007