3SV Blow Down Valve leaking air on an Ingersoll Rand rotary air compressor

What is the purpose of this blow down valve? As of right now, it is leaking air and oil out of it. At a certain pressure (when air drops below the max of 150 psi), it stops leaking. I did replace this part at one time because it did not stop leaking air. I know that this use to not happen when the unit was first purchased. Let me know what you think. Thank you.