If "receive disable string" is MAINTENANCE and if the /service_status.html page is serving MAINTENANCE, then the pool member should not be UP.
See this SOL13397 for the right formatting.
An example:
GET /index.html HTTP/1.1\r\nHost: host.domain.com\r\nConnection: Close
If you are still running into issues after fixing any formatting issues, I would recommend running curl from the F5's bash shell. Example:
curl -H "Host: domain.com" http://10.10.10.10/service_status.html
I am assuming that the IP of the pool member is 10.10.10.10 and that it is listening on port 80. This will help you to identify the response that the F5 is seeing.
If the pool member is listening on a different port, you would have to configure the HTTP monitor to check the pool member on the right port.