Cast character to boolean

I'm currently refactoring a database that somebody else designed.
When the database was designed he used character columns with a length
of 1 char to represent some values that really should have been
represented as booleans.  He used 'y' for true and 'n' for false.

I want to cast these columns into the correct type, because you could
in theory set the columns in question to any single character value.
I don't seem to be able to do so, however, the database keeps claiming
that the cast cannot be done.

I tried casting the columns in question to character varying and then
changing all the 'y's to 'TRUE's, and all the 'n's to 'FALSE's. This
wasn't a problem.  But casting from this format to boolean still gives
an error.

Does anybody know how to do this?