iRule to mark pool member down if HTTP Status Code 502/503

I'd suggest that using a HTTP health monitor would be more appropriate. In this case perhaps a second monitor configured for 'reverse' mode so the Pool Member is marked up unless a 502 is received; you can configure it to mark down after 5 occurances using the appropriate timer values. Then you'd just need to add the monitor to the Pool and configure it to mark down if either monitor fails.