SELinux and "su -"

When logging in via ssh, then su - to root user. I am in the wrong
context. I stay in as user_u:user_r:user_t rather than moving to
root:staff_r:staff_t. I would figure that root would go into its
default context then I would be allowed to newrole -r sysadm_r to handle
sysadmin tasks. Instead, when trying to to newrole -r sysadm_r, I get
the error user_u:sysadm_r:sysadm_t is an invalid context -- which I
would expect since I am still user_u after su'ing over. Is there a way
to get around this particular problem. I do not want to give user_u the
ability to have a sysadm context. I would rather I could get the actual
"root" context when I su over.

Hope that made sense. I'm new to SELinux (on Gentoo FYI) and trying to
get my head around the whole administration of it.

TIA

Ken