Pardon but I don't understand the question... ALL vac is negative pressure. If refering to the pcv it's job is to take the negative pressure in the carb area and allow the pressure in the valve cover to leak into the intake.
The OTHER hose on the valve cover is there to supply fresh air to the valve cover in newer engines. (ones that don't yet have blowby) If an engine has too much blowby for the pcv to handle The second hose can be attached to a vac source and between the TWO hoses the blowby should be taken care of.
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />