You can install CPL into your local policy file to prevent the ProxySG from caching an HTTP 503 return code from an internal web server. For information on how to install CPL into your local policy file, please see 000010101. Here is the CPL to install:
; **BEGIN** CPL code to not cache 503 errors returned from host.example.com
url.domain=host.example.com http.response.code=503 cache(no)
; **END** CPL code to not cache 503 errors returned from host.example.com
If there are other error codes that you want the proxy to not cache, then you can add additional url.domain lines with different error codes.