Pics would help, but consider drawing the pond down and using hydraulic cement to seal the wet face of the dam - a tuck-point job, so to speak.
Like others have said, pictures would help tremendously, I am having a hard time picturing just what you have going on, it would take a pretty wide, substantial block dam to withstand much water pressure.
Pictures can be a little cumbersome to get to show up on the site tho till you get the hang of it, there is a thread or two on the subject, look for the more recent one as it changes a little from time to time.
