How can I configure mod_proxy not specifically not cache 503 errors?
mod_proxy marks an unresponsive backend as down when it seems to be down; if no backends are available then it responds with a 503.
By default, a down backend will be marked as such for 60 seconds; until that time passes, it won't retry the connection (and replies with an error message to any connecting clients).
To have it retry immediately, add retry=0 to your ProxyPass directive:
ProxyPass /manage/ http://localhost:9000/manage/ retry=0