setting up a maintenance page using LB_FAILED

The problem is due to the fact that you don't want to use LB_FAILED for this. You want something in the HTTP_REQUEST section that checks the number of active pool members.

LB_FAILED only fires if load balancing fails (I.E. no members available, no fallback entry, etc. etc.). You can read more about it here: Click here

Try something like:

when HTTP_REQUEST {
  if {[active_members myHTTPPool] < 1 } {
    HTTP::redirect "http://www.yahoo.com"
  }
}

HTH,

Colin