ActionResult nie zwraca typow operacji

Witam mam taki problem,
na stronie po zaznaczeniu checkboxów zwraca mi w modelu null.

Model

public class TReportsViewModel  : GridViewModel
{
     public List<COPerType>          OprType = new List<COPerType>();
}

 public class COPerType
    {
        private bool    bCheck;     // checked
        private string  bName;      // name operation

        #region getters/setters
        public bool Check
        {
            get { return bCheck; }
            set { bCheck = value; }
        }  
        public string Name
        {
            get { return bName; }
            set { bName = value; }
        }
    }

Controller

public ViewResult ActivityOpt()
		{
			TReportsViewModel tm = new TReportsViewModel();
			string[] sOperationTypes = { "otwarcie nagrania", "eksport nagrania"  };
			tm.bAllOper = true;
			for( int i = 0; i < sOperationTypes.Length;i++ )
			{
				COPerType opt = new COPerType();
				opt.Check = false;
				opt.Name = sOperationTypes[i];
				tm.OprType.Add( opt );
			}
			return View("~/Views/Reports/ActivityOpt.cshtml", tm);
		}
 [HttpPost]
public ActionResult ActivityOpt(TReportsViewModel tvm)
{
        
}

View

@using( Html.BeginForm() )
{
    @Html.ValidationSummary( true )

            <table width="160" cellpadding="0" cellspacing="0" border="0" style="border:none">
                @for( int l = 0; l < Model.OprType.Count;l++ )
                {
                    <tr style="border:none">
                        <td style="border:none">
                            @Html.CheckBoxFor( x => x.OprType[l].Check )
                            @Html.DisplayFor( x => x.OprType[l].Name )                   
                        </td>
                    </tr>
                }
            </table>
    <input type="submit" value="@Resources.OK" style="margin-left:0px;position:absolute"/>

}

po wysłaniu formularza mój model OprType = 0. Nie zwraca mi zaznaczonych checkbxów.

Z góry dziękuje i pozdrawiam
PiK