That answer help me: Why asp.net validators prevent all other postback?
I've made a ValidationGroup:
Added the property ValidationGroup="AddBeerValidationGroup"
to each CustomValidator
Partial modified Code behind:
protected void Button1_Click(object sender, EventArgs e)
{
Page.Validate("AddBeerValidationGroup");
if (Page.IsValid == true)
{ /* SOME CODE ..... */ }
}