What if safety valves got broke or upregulate? How would you know.
I remember that one of pumping vets on this board checked the vacuum created by one of those ‘medical designed’ vacuum pumps, without a gauge as well; found that the pressure generated could reach way more than what is considered safe and effective in the long run. I think for that price you can buy a better pump with a gauge. Anyway you can ask to vets with more knowledge of this field, like Avocet or Gprent; if you don’t get an answer here try pming them.