How to create RunningValue Through Column Groups?

Hi Everybody i´m trying to do this with a Matrix

I want the running value of the product quantity for each month of a year, resetting the running value

when a new year is reached .

I have 2 column groups "Year" and as it´s child group "Month".

                     2009                                   2010

                     January Febrary March      January   February  March

Product

Toolbelt            10           15       20              1            10            50

Well that is the idea...

In the field of the quantity i´m using an expression like this = RunningValue(Fields!Quantity.Value, Sum, Nothing) .

With this expression i get the the running value and it´s great, but it does not reset thru the years ,

it continues aggregating the quantity value.

So i tried with this expression assuming if i set the SCOPE to the Year group i can achieve this

= RunningValue(Fields!Quantity.Value, Sum, "Year") ,

But surprise!!, this is not giving me the right aggregation , i´m still dig in it and can´t find what it actually aggregates.

Am I doing something wrong?.

All the help is welcome. Thanks.