You should be using the yellow spring. The default peak boost of a BK2 should be around 20psi. Based on your description it sounds like you are leaking boost pressure through the BOV.
When I had the forge I used the yellow. I was tuned for 22psi at the time and had no issues with it.
When I had the forge I used the yellow. I was tuned for 22psi at the time and had no issues with it.