Output Caching In HTTP Handler And SetValidUntilExpires?

I'm using output caching in my custom HTTP handler in the following way:

public void ProcessRequest(HttpContext context)
{
TimeSpan freshness = new TimeSpan(0, 0, 0, 60);
context.Response.Cache.SetExpires(DateTime.Now.Add(freshness));
context.Response.Cache.SetMaxAge(freshness);
context.Response.Cache.SetCacheability(HttpCacheability.Public);
context.Response.Cache.SetValidUntilExpires(true);
...
}

It works, but the problem is that refreshing the page with F5 leads to page regeneration (instead of cache usage) despite of the last codeline:

context.Response.Cache.SetValidUntilExpires(true);

UPD: Seems like the cause of problem is that HTTP handler response isn't caching on server. The following code works well for web-form, but not for handler:

Response.Cache.SetCacheability(HttpCacheability.Server);

Are there some specifics of the caching the http handler response on server?