Jboss and largeObjectManager

I am using Jbos 4.0 and postgresql 7.3 as database. As datasource I have
pooled connection (local-tx-datasource).

As long as I do not work with large object it seems to be fine.

But how do get an instance of the LargeObjectManager.
When I try to do

LargeObjectManager lobj = ((org.postgresql.PGConnection)
dbcon).getLargeObjectAPI();

I get the error java.lang.ClassCastException:
org.jboss.resource.adapter.jdbc.WrappedConnection

I hope someone can help me.

Best regards

Torben Froeberg