You should consider using the cleanup command in your recovery.conf file, this will make sure that wal files no longer needed by the secondary server are eliminated.
If you need the wal files for PITR, yo could also setup your archive command to rchive the wal files to two different locations. a shared location where the secondary can read if it needs to catch up and a keep location which is used by the primary just for PITR which you could then clean up after each base backup.
In case you ask, if this is a unix box, you can separatethe two commands for archive_command with && for example
comand 1 && command 2
JT