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

You can additionally specify a receive disable string something like "503". The monitor will disable the member if it catches the receive disable string and will mark the member down if it doesn't catch the receive string. Just specifying a receive string should account for your situation though where you want to mark the member down if you aren't receiving a 200.

http://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/ltm_monitors_reference_11_0_0/2.html

Check 'Creating a Custom HTTP Monitor', steps 11-13.