Get updated values in Web Form from a value in the Master Page using ModalPopupExtender

Hi guys, this is my first post here so nice to meet you all.

I'm new working with ASP.NET and AJAX as well, so i will try to explain myself as best as i can. I have created a Web Form from a Master Page, wich has a button that calls a function to show the ModalPopupExtender in which i have two buttons and it works fine, but what i need now is to 'capture' the pressed button and then pass the value to the original form. I can capture the pressed button, but the Web Form won't update and it doesn't shows the value. I have to click on the button that calls the ModalPopupExtender again and then it shows tha value from the last pressed button, but i need to have it before that.

Firstly, this is where i make the Modal in the MasterPage:

<asp:UpdatePanel ID="UpdatePanel1" runat="server"><ContentTemplate><asp:Panel ID="pnlMensaje" runat="server" CssClass="CajaDialogo" style= "display: none;"><asp:Panel ID="mensajeheaderPanel" runat="server"><table id="mensajetable" width="430px" style="margin: 0px; padding: 0px; height:auto;"><tr><td align="left" valign="middle"><asp:Label ID="headerLabel" runat="server" Text="Message Information" Font-Bold="true"/></td><td valign="middle" align="right"><asp:ImageButton ID="cerrarmensajeImageButton" runat="server" ImageUrl="~/App_Themes/Popup/img/dialog_close.gif" Width="14px" ToolTip="Cerrar mensaje" /></td></tr></table></asp:Panel><table align="left" style="margin: 0px 0px 0px 0px;"><tr><td><asp:Label ID="mensajeLabel" runat="server" Font-Size="13px" Text="Datos de usuario incorrectos" /></td></tr></table><br /><br /><br /><br /><br /><br /><table id="Table1" width="100%"><tr><td align= "center"><asp:HiddenField ID="valBtnOk" runat="server" /><asp:Button ID="btnOk" runat="server" Text="Save" Width="109px" /></td><td align= "center" abbr="   "><asp:HiddenField ID="valBtnCancel" runat="server" /><asp:Button ID="btnCancelar" runat="server" Text="Cancel" Width="109px" ToolTip="Cerrar mensaje" /></td></tr></table></asp:Panel><asp:ModalPopupExtender ID="mpeMensaje" runat="server"
                                                BackgroundCssClass="FondoAplicacion" OkControlID="cerrarmensajeImageButton"
                                                PopupControlID="pnlMensaje" TargetControlID="lblOculto"></asp:ModalPopupExtender><asp:Label ID="lblOculto" runat="server" Text="Label" Style="display: none;"> </asp:Label></ContentTemplate></asp:UpdatePanel>

Then comes the function to show the Popup:

Public Sub ShowPopUp(ByVal m As String, ByVal type As Integer, Optional ByVal h As String = "Message Information")
        Select Case type
            Case 2
                pnlmns = "CajaDialogoAdvertencia"
                pnlheader = "MensajeHeaderAdvertencia"
                lblOK = "OK"
                botones = False
            Case 1
                pnlmns = "CajaDialogoCorrecto"
                pnlheader = "MensajeHeaderCorrecto"
                lblOK = "Save"
                botones = True
            Case 0
                pnlmns = "CajaDialogoError"
                pnlheader = "MensajeHeaderError"
                lblOK = "OK"
                botones = False
        End Select
        mpeMensaje.Show()
        msn = m
        hdr = h
    End Sub

This only assigns the css styles selected for each button and the message to show. I do all this with WriteOnly propertys like:

WriteOnly Property pnlmns() As String
        Set(ByVal value As String)
            pnlMensaje.CssClass = value
        End Set
    End Property

The buttons code is like this:

Protected Sub btnOk_Click(ByVal sender As Object, ByVal e As EventArgs) Handles btnOk.Click
        valBtnOk.Value = 1
        valBtnCancel.Value = 0
    End Sub

Where valBtnOk and valBtnCancel are Hidden Fields to 'capture' the pressed button.

And finally, i call this method from my WebForm in the button click method:

Protected Sub btnSuccess_Click(ByVal sender As Object, ByVal e As EventArgs) Handles btnSuccess.Click
        m.ShowPopUp("Error Message", 1, "Success!")
        If m.resBtnCancel = "1" Then
            lblRes.Text = "Cancel"
        ElseIf m.resBtnOk = "1" Then
            lblRes.Text = "OK"
        End If
    End Sub

The problem is, it doesn't 'wait' for the popup to make a desicion before entering the 'If', thus is doesn't get the value until the next click. By the way, the values from the buttons i get them with a ReadOnly property with this code:

ReadOnly Property resBtnOk
        Get
            Return valBtnOk.Value
        End Get
    End Property

I have tried the Javascript method of adding:

function fnClickOK(sender, e) {
            __doPostBack(sender, e);
        }

and then on the load method put this: 

btnOk.OnClientClick = [String].Format("fnClickOK('{0}','{1}')", btnOk.UniqueID, "")

but that only 'reloads' the Master Page, not the original form.

This is the final step i need to get this working, so if anyone can help me i would really aprecciatte it. Thanks in advance