Page Post Back But Server Click Event Doesn't Fire

I have a button like this:

<asp:Button ID="btnSave" runat="server" ClientIDMode="Static" Text="Save" OnClientClick="return ConfirmSave();" OnClick="btnSave_Click" />

If I write my client function like the following, it works as expected:

function ConfirmSave()
{
return confirm('Confirm?');
}

But I should check, inside the function, for the confirm result, like this:

function ConfirmSave()
{
if (Page_ClientValidate('validationGroup'))
{
var conf = confirm("Confirm?");
if (conf)
{
$('#btnSave').attr('disabled', 'disabled');
}
return conf;
}
else
return false;
}

Doing this, the page postback but the server click event doesn't fire.