There are also two types of "Knobs" used on air lines.
1) Regulator valve knobs
2) Shutoff valve knobs
Both can reduce the air flow, but there is a big difference. The regulator is a special device that you set to a specific air pressure. The regulator will then open or close the valve from fully open to fully close as is needed to keep the output pressure at the setting you set.
The second is just an on/off valve that controls the size of the hole between the two sides. When there is air flowing through a tool, this type of valve will restrict that flow, but when the tool stops, air can still flow because the size of the valve hole does not change. So the air will continue to flow till the PSI at the tool is the same as the PSI at the compressor.
So the regulator limits the max pressure exiting the valve, while the shutoff valve limits the max air flow through the valve.
The question is do you have a a regulator valve or a shutoff valve. ___________ Good point!
