btmp not logging

[jimm]

Hi all,
I have inherited (came with the job) a CentOS 4.6 mail server. Everything ran fine until I copied the btmp file (64MB) to btmp.1 and cat /dev/null>/var/log/btmp. After this move btmp doesn't log anything. Wtmp is logging just fine. I'm not familiar enough with the btmp process to know what drives the logging or where to start looking for problems. BTW bad logins are put in /var/log/messages by syslog. Any insight would be greatly appreciated.
Jim