Problem with cache and max-age

Hi friends, I've a problem and need to know if can you help me, thanks in advance. I used the libwww in order to implement the cache features, but it don't work correctly because when the page has expired, it continue using the cached file and don't use the remote file. I set the request cache options as follow:

HTRequest_setReloadMode(m_request, HT_CACHE_VALIDATE );
HTCacheMode_setExpires(HT_EXPIRES_AUTO);
HTRequest_addCacheControl(m_request, "must-revalidate", ""); 
HTRequest_addCacheControl(m_request, "cache-control", "max-age");
HTRequest_addCacheControl(m_request, "max-age", "30"); //seconds

However, it don't work :-( 

Thanks,

Naldo Alcalde H.