iRule - Down node using Response Header

Have you considered doing this in an HTTP monitor?

ltm monitor http HTTP-Down_on_responce_header {
    defaults-from http
    destination *:http
    interval 5
    ip-dscp 0
    recv "x-node: up"
    recv-disable "x-node: down"
    send "GET /\r\n"
    time-until-up 0
    timeout 16
}