WP7 (windows phone 7) HttpWebRequest losing POST data

I'm sending a lot of POST data (over 5000 chars) and it seems that WP7 HttpWebRequest is losing some of the content in the process...

The data is a png converted to a byte array and after to a base64 string.

If my data is 4941 characters long, it will send only 1448 chars, the content length (verified with wireshark) of the request is fine (Content-Length: 4991).

How i send the data :

void SendPost()
{
    // Create the web request object
    HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create(url);
    webRequest.Method = "POST";
    webRequest.ContentType = "application/x-www-form-urlencoded";

    // Start the request
    webRequest.BeginGetRequestStream(new AsyncCallback(GetRequestStreamCallback), webRequest);

}

void GetRequestStreamCallback(IAsyncResult asynchronousResult)
{
    HttpWebRequest webRequest = (HttpWebRequest)asynchronousResult.AsyncState;
    // End the stream request operation
    Stream postStream = webRequest.EndGetRequestStream(asynchronousResult);

    // Create the post data
    // Demo POST data (length = 3762)
    string postData = "data=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