The OPV valve only opens when the pressure in the chanmer or line to which it is connected meets or exceeds the force needed to push the spring back and open the waste pathway. If your grind is too coarse or the distribution is poor, and the water can pass trough the coffee at a lower pressure than the set pressure of the OPV, then no, it will not open. Just because e pump is capable of producing 15 BAR does not men that it produces 15 BAR every time you turn it on.QUESTION:
Shouldnt the OPV operate during EVERY SHOT on espresso machines with vibe-pumps? Since theyre rated much higher than needed for pulling shots.