Return status code and a retry after X value

When you say it doesn't work, do you mean that the Retry-After header isn't set in the response? Else, can you try testing with curl from the LTM command line:

curl -v --user-agent "google" http://1.1.1.1/

Can you replace the colon with a space in Retry-After 10800:

when HTTP_REQUEST {
   HTTP::respond 503 noserver Retry-After 3
}

> GET /test.123 HTTP/1.1

> User-Agent: curl/7.15.5 (i686-redhat-linux-gnu) libcurl/7.15.5 OpenSSL/0.9.8b zlib/1.2.3 libidn/0.6.5

> Host: 7.7.7.18

> Accept: */*

>

< HTTP/1.0 503 Service Unavailable

< Retry-After: 3

< Connection: Keep-Alive

< Content-Length: 0

Aaron