Same here, however I've noticed that it's safe to pulse up to 10Hg for a few seconds before reducing the pressure to 5Hg and below. High pressure sessions of 5Hg and above which last longer than 20 minutes usually cause blood spotting and a donut will start to form if you have a foreskin.
Just for info, I spent 6 months pumping at around 5Hg for 1-2 hours, 3 times per week. I ended up with serious discolouration on my shaft which took a full year to disappear.
If I can't maintain an erection, pressures below 5Hg don't succeed in keeping my cock at maximum expansion, I merely maintain an engorged state, some people claim that this can still result in growth but I like the temporary enlargement of a more intense session.