Behavior-ViewModel Binding

I'm trying to write a simple behavior with max- and min- values to validate an entered value within a certain range:

<Entry Text="{Binding Quantity, Mode=TwoWay}">
  <Entry.Behaviors>
    <ui:ValueInRangeValidator x:Name="QtyValidator" MinValue="1" MaxValue="10"/>
  </Entry.Behaviors>
</Entry>

This works fine, but when trying to add binding eg. MaxValue="{Binding MaxVal}" the value is left as it's default.

I'm using the following in my behavior:

public static BindableProperty MaxValueProperty =
        BindableProperty.Create("MaxValue", typeof(decimal?), typeof(ValueInRangeValidator));
public decimal? MaxValue
{
    get { return (decimal?)GetValue(MaxValueProperty); }
    set
    {
        SetValue(MaxValueProperty, value);
        OnPropertyChanged();
    }
}

I've also tried setting the property changed event on the BindableProperty but it never seems to fire. I know the viewmodel is correct as binding the same property to a label displays the value.