you said that the reservoar is full of bubbles?
technically is foaming and it cause by air in the system.
and you said that the resevoir is full. The reservoir should not be full but at the marking at "full hot" when engine is hot, and at "full cold" when engine is cold.
If the reservoir is completely full like you said, that could ,or have allready damage the pump or gear.
Mechanic On Wheels LLC