Taking a total stab in the dark:
Have you tried
proxy_cache_min_uses 0;
?
Also I assume your back end is sending the appropriate caching headers? From the doco:
The following response headers flag a response as uncacheable unless they are ignored:
Set-Cookie
Cache-Control containing "no-cache", "no-store", "private", or a "max-age" with a non-numeric or 0 value
Expires with a time in the past
X-Accel-Expires: 0
Have you tried
proxy_cache_min_uses 0;
?
Also I assume your back end is sending the appropriate caching headers? From the doco:
The following response headers flag a response as uncacheable unless they are ignored:
Set-Cookie
Cache-Control containing "no-cache", "no-store", "private", or a "max-age" with a non-numeric or 0 value
Expires with a time in the past
X-Accel-Expires: 0