Disk space consumed by pk not returned after vacuum or reindex

Hello,

I am running into a problem on a RHEL3 systems, running PostgreSQL 7.4

We have a database, which consistently consumes more and more of the
disk space in it's lvol until it reaches 100%. So far, we have tried to
run a full vacuum on the database, with limited success. Eventually, we
had to drop and reload the database with the same data inside. It
brought the disk usage down to 73%. It then began to creep once more
toward 100%. After some research, I was able to use the pg_class
catalog to find that the items which are expanding the quickest are
primary key (btree) indexes. I attempted to run a REINDEX on one of the
tables with the pk taking up the largest amount of space. The usage
according to pg_class dropped dramatically, however the disk space was
not returned to the system. So I attempted another full vacuum
afterwards, and still nothing was returned to the system. These tables
are updated extremely frequently (although their total number of rows is
close to constant), which is my guess as to why the pk indexes increase
so rapidly in terms of their disk usage. Unfortunately, PostgreSQL
knowledge is limited, and I was wondering if anyone had experienced
something similar / knows what else we can do to return this disk space
back to the system?

Thank you in advance for any/all help!

Kevin