Browser Back Button Issue in asp.net

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ASP.NET

I'm having trouble with a simple ASP.NET application and the back button after a post back.

The page in question has a simple form on it, some text fields etc, and a dropdown that does a postback (autopostback).

The "normal" flow is the user fills out the form and perhaps changes the dropdown. Based on the dropdown value the page content might change.

The problem I'm having is that after the user has changed the dropdown and the postback has completed then the user clicks the back button. They see a "webpage has expired" message from IE.

I've set the following:

Response.Cache.SetExpires(DateTime.Now.AddMinutes(-1));
Response.Cache.SetCacheability(HttpCacheability.Private);
But that doesn't seem to have nailed the problem.

The actual Cache-Control response header reads as: private, no-cache:"Set-Cookie"

In a classic ASP application the with a Cache-Control response header of just "private" the back button behaves as expected after a "post back".

Is there anyway to force ASP.NET to set the cache-control explicitly to exactly "private"? Or any other solution that results in the back button and postbacks working well together?

Thanks!