That port is the recommended place for no pressure return. That isn't your problem. Problem may be the type of selective control valve you have. The standard valve for 6420 is the 200 series valve and they are not designed to have flow compensation when other functions are used. The optional 300 series valve is what you need. The cap on top of the valve is where you adjust your detents. DOes your valves have 2 position or 3? You need the 3 position valve to operated consistent speed for your vacuum. If you do have 300 series valves, then you may have a sticking valve or broken spring in compensator valve.