Wow, as a former cock and ball vac pumper (don't laugh everybody)
you have some serious bruising happening. There are guys that do vac on breasts on newart.com although most are saline infusion.
The technique is low pressure with long duration. Also If you can, wet pumping (what I used to do) with warm water producies better results. giving that some thought, I'd start with by boobies wet, some hot water in the cup, lay on my back so water sinks in cup.pump to get a good seal, hold 15~30 minutes. Remove and massage with olive oil or coconut oil for a 5~10 minute break. then back in cups for another 30 minutes.....no over pressure, just a good pull. Watch for broken capillaries.
Remember water, low pressures, regular breaks.....all will produce better results.
And no....I quit pumping long ago ....but hey , my experience came in handy. Always managed to get my "package" pretty big with no damage.