If there is a slow leak of pressure, yes, keep pumping it back up to 5 inHg. As long as the glans is acting as an indicator that the additional pressure you are using to get the vacuum back up is not a problem. Going past the 8in tube marker is a marker that the suction is spot on, pulling and tugging on your scrotum and inner penis up and up, including stretching the living daylight out of the penis shaft. Just make sure the residual pressure in the tube is dropping as well and not accumulative from your additional pumping. Not good if you're subjecting your glans to too high of a total pressure. This is where blisters will form.
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