Logging response data

I already know about HTTP::collect and HTTP:payload. My current iRule looks like this:

when HTTP_REQUEST_DATA {
    set request_payload [HTTP::payload]
}


when HTTP_RESPONSE_DATA {
    set response_payload [HTTP::payload]
    HSL::send [HSL::open -proto UDP -pool syslog_server] "HSL TEST: BLA"
}


when HTTP_REQUEST {
    set request_payload ""
    set response_payload ""
    set client_ip [getfield [IP::remote_addr] "%" 1]
    set http_method [HTTP::method]
    set port [TCP::local_port]
    
    if { $http_method eq "POST" } {
        if { [HTTP::header Content-Length] != "" } {
            HTTP::collect [HTTP::header Content-Length]
        } else {
            HTTP::collect 1048576
        }
    }
}


when HTTP_RESPONSE
{
    if { $http_method eq "POST" } {
        if { [HTTP::header Content-Length] != "" } {
            HTTP::collect [HTTP::header Content-Length]
        } else {
            HTTP::collect 1048576
        }
        
        HSL::send [HSL::open -proto UDP -pool syslog_server] "HSL TEST:\n\nClient: $client_ip\n\nRequest Data: $request_payload\n\nResponse Data: $response_payload"
    }
}

The problem is that the "HSL TEST: BLA" will never be logged because the event HTTP_RESPONSE_DATA gets never called (thats what I think).

It actually makes no difference if HTTP::collect is used in HTTP_RESPONSE or not.

Any idea anyone?