HttpWebResponse Do not pumped bytes from the stream BinaryReader

Question

I use this method in System.net.HttpWebResponse. But I can't get all the bytes into a variable, what do I do wrong?

    [System.Net.HttpWebRequest] $webRequest = [System.Net.WebRequest]::Create('URl')
    $webRequest.Timeout = 300000
    $webRequest.Method = 'POST'
    $webRequest.ContentType = 'multipart/form-data'
    $webRequest.ContentLength = $data.Length;
    $requestStream = $webRequest.GetRequestStream()
    $requestStream.Write($data, 0, $data.Length)
    $requestStream.Flush()
    $requestStream.Close()
    [System.Net.HttpWebResponse] $webResponse = $webRequest.GetResponse()
$streamReader = New-Object IO.BinaryReader($webResponse.GetResponseStream())
$ByteArr = $streamReader.ReadBytes($streamReader.BaseStream.Length)

Why does $streamReader.BaseStream.length have no length? When I use the $streamReader.ReadByte() method from the stream pumped bytes. There is a contradiction?