postgresql question

Hello:

I am running Postgres version 7.4.13 on a Linux CentOS box with kernel
version 2.6.9-67.0.7
I have a process running under a user ( "merlin" ) that needs access to the
data base ( to calculate the DB size ). I have added the "merlin" user as a
member of the postgres group, and changed the file group permissions ( I.E.
/var/lib/pgsql/data/base ) to 750 from 700. this allows the "merlin" user's
process to access the DB files. However - and this is the problem - after
changing the file permissions, the Postmaster won't start. The indication
is that it does'nt know where to find the database system data. I provide a
path ( using the -D option ), and then the indication is that it can not
read the permission of the directory /var/lib/pgsql/data/base/171142. If I
do nothing other than remove the file permissions ( change back to 700 from
750 ), the Postmaster starts just as it should. Why does the Postmaster not
start with the group permissions changed? Is this a bug? What can I do?

TIA
Dennis