How do I detect if a pool is down in an iRule?

I am trying the follwowing rule

rule sorry_test {
   when LB_FAILED {
       log local0.err "ERR: Load Balancing failed for [virtual name]  Pool: [LB::server]  Client: [IP::remote_addr] 
       HTTP::redirect "http://sorry.domain.com/sorry.html"
   }
}

Now when I make a request to the virtual immediately after shutting down the webserver I get an empty reply from the server and I get the following entry in /var/log/ltm:

Mar  5 16:47:02 tmm tmm[1028]: Rule sorry_test : ERR: Load Balancing failed for testvip  Pool: test_pool  Client: 10.1.1.1 
Mar  5 16:47:02 tmm tmm[1028]: 011f0007:3: http_process_state_header_xfer - Invalid action EV_SINK_HEADER during ST_HTTP_XFER_HEADERS

So it appears to have triggered the LB_FAILED event but the HTTP:redirect did not get executed.

Then a few seconds later when the monitor marks the pool member down, I get the correct redirect the response to my HTTP request and I get the following in /var/log/ltm

Mar  5 16:47:03 testbip mcpd[1119]: 01070638:3: Pool member 10.2.2.2:80 monitor status down.
Mar  5 16:47:04 tmm tmm[1028]: Rule sorry_test : ERR: Load Balancing failed for testvip  Pool: test_pool  Client: 10.1.1.1
Mar  5 16:47:04 tmm tmm[1028]: 011f0007:3: http_process_state_header_xfer - Invalid action EV_SINK_HEADER during ST_HTTP_XFER_HEADERS

In this case, the LB_FAILED event was triggered and the HTTP::redirect was executed.

So my questions are:

1. Why does the HTTP::redirect not get executed in the first instance?

2. What does the EV_SINK_HEADER error message mean?

-Al