Pictures would help.

I don't get a clear sense of the problem with the back lever. Is the head of the brake cable not seating correctly inside the lever? Inside the brake lever is a barrel shaped piece (cable anchor ) into which brake cable seats. Is that the part that is missing? If you think the calipers themselves are ok, purchasing replacement levers is certainly an option since you apparently are missing one anyway. Installation is not hard but adjusting single pivot calipers, which I assume you have, is a bit of a challenge. You might want to consider upgrading to dual pivot calipers, which have more stopping power and are easier to keep centered and adjusted properly.

Here is a link to park guide on calipers. It contains a further link to discussion of levers themselves.

http://www.parktool.com/blog/repair-...-brake-service