Irule to Make a Member-server down based on error: 503

It may be best to focus your send string based on a successful monitor attempt. Ideally you want the send string to get something reflective of the actual health of the server/application.

If the application is healthy, then you can use curl to run a query against it to see how it responds. Then build your send string around that response. This ensures that if the server returns anything other than what's expected (whether it's a 503 or whatever), then the health of that server is appropriately reflected by LTM. If you are only matching on a 500 level code, then you may not be properly reflecting the health of the server/application.