priority LB - maybe better suited to iRule?

Since you don't want persistence when the priority 1 nodes are used, this is likely a case where you'll want to instead split up your priority 1 and priority 2 nodes into different pools.

Then put the priority 2 pool as the default pool on the virtual server and write an iRule that looks like this:

when LB_FAILED {
   if { [LB::server pool] eq prio_2_mbrs and \
        [active_members prio_2_mbrs] == 0 } {
      pool prio_1_mbrs
      persist none
   }
}

What will happen is that by default the priority 2 nodes will be used. If it fails to connect to a pool member from that pool, it will check whether it was because there are none availabe and switch to the priority 1 pool, disabling persistence.

Hope that helps.