Normally PCV manifold were have vacuum air pressure in low rpm(PCV valve working during idling), however went come to acceleration in high RPM then most air intake come from the air filter from car throttle, then there were no more air vacuum in PCV manifold.
I made mistake too when first time connect the my DIY OCT too. Please refer to the picture. There normally to tube on the engine cover one was the waste air breath, the other was the PCV manifold. However different car may have different structure, refer to car mechanic if you not sure.