Why do you need a pressure cap to make a puke tank work? If there is no other source for air to return to the system when the engine cools down, and the end of the tube is below the surface of the liquid in the catch tank, the coolant will have to siphon back into the system. All a pressure cap does is raise the boiling point, 3 degrees per pound. It helps if the catch tank is close to the same vertical level as the overflow so the suction won't have to lift the coolant very far to get it back into the radiator.
Jerry
