priority LB - maybe better suited to iRule?

Ok. Sorry about that. To add logging use the "log" statement.

So, for example:

when LB_FAILED {
   log local0. "failed for pool [LB::server pool], active mbrs = [active_members [LB::server pool]]"
}

This will generate a log message into syslog that will end up in /var/log/ltm.

Now, on to your problem. Just a hunch but try adding parenthesis () around the two parts of the expression in your if:

if { ( [LB::server pool] eq "main_pool" ) and ( [active_members main_pool] == 0 ) } {

If this fixes it, try searching for "operator precendence" and you should find some reasonable explanation that I've already made elsewhere on devcentral.