[Bacula-users] Volume truncation not working

Hi all, I'm running Bacula 5.0.3 on ArchLinux 64-bit with mysql 5.1, backing up to disk volumes. I've purged a number of volumes using commands like purge volume=Vol-0008. The problem is that the files on disk are not getting truncated. I've tried both commands I saw in the manual for this

[Bacula-users] When are volumes marked Purged?

Minor background: Bacula 5.0.3, ArchLinux 64-bit, mysql 5.1. I'm doing my backups to disk, and have my filesize limited to 50G. Earlier, it was set to 5G, and this discussion concerns volumes from that earlier setting. I have several volumes which were all written by the same full backup job.