[squid-users] Do not make conditional requests to the originserver when cache hits. Is it possible?

Hello,

I just setup a reverse proxy. But I noticed squid always send
conditional requests to the backend server even its local cache
contains the fresh data. The response headers looks like this:

--
HTTP/1.0 200 OK
Last-Modified: Wed, 17 03 2010 08:26:15 GMT
Cache-Control: max-age=86400
Content-Type: text/plain
X-Cache: MISS from ku-office
X-Cache-Lookup: HIT from ku-office:81
Via: 1.0 ku-office:81 (squid/2.7.STABLE8)
Connection: close
--
The backend server listened on port 80 and squid listened on port 81.
And my settings of squid are:
--
http_port 81 vport=80 defaultsite=localhost
cache_peer 127.0.0.1 parent 80 0 no-query no-digest originserver
refresh_pattern .		0	20%	4320
--
I'd like to let squid make the conditional request only when the cache
entry is expired. What am I missing?
Thanks.
-- 
Kai-Chieh Ku