auto vacuum doens't appear to be working

We have a bytea column where we store large attachments (ie pdf file).
every so often (2 weekly) we replace the large a attachment (5-15mb) with a
much smaller binary file (15k).

when I run
SELECT sum( relpages*8/1024 ) as MB FROM pg_class where relname != ''
I get a value of 104995 which I interpret to mean I have 104GB of stored data
in the database and this value has remained relatively static (+/- 1GB) over
the past couple of weeks.
We I to a df -h on the filesystem holding the database cluster I get a usage
of 140GB. Again I interpret this to mean I have nearly 35GB of "uncleaned"
data.

Is this a case where I should be running the vacuum manually or is auto vacuum
all that should be necessary to keep track and mark the updated tuple space
ready for re-use.

thanks

--
Warren Little
Chief Technology Officer
Meridias Capital, Inc
1006 Atherton Dr
Salt Lake City, UT 84123
ph. 866.369.7763