Postgresql & Redhat 6.2 = memory leak?

I am running postgresql 7.1.2 on Redhat 6.2 with 387 megs of memory. After
running a vacuum analyze or loading postgresql with 50 megs of data. The
memory drops down to only a couple of megs.  After stopping postgresql the
memory does not return.  Has anyone had this problem before?

thanx..

Mike