To see if I'm understanding what you're saying, let me try to rephrase it.
If the pump is made for liquids (a much less compressible medium), then the seals for the check valves may have a reverse breakdown at a lower pressure with gas (a much more compressible medium). This being due to the seals not being as strong.
That makes sense to me. I figured that the way check valves function this wouldn't be the source. However, less effective seals around the valve would explain it somewhat.
Yes, that is what I'm talking about.
So the actual use of this pump in the machine is a vacuum, which involves different forces than compression (if I'm understanding correctly). According to my tests, that spec seems to match. However, I did also save a separate check valve that was somehow connected to this pump, as well as a smaller pump (the actual name of it escapes me currently). It doesn't seem to create enough negative or positive pressure to overcome this valve, but I may be overlooking how it was implemented. I vaguely remember how it was attached to the machine. Different tubes stemmed off a 4 output plastic tee, one of which has the check valve on it.
Although it may have not been your intent: I thank you for giving me somewhere to look. Although it probably should have been obvious: if I can find some schematic of how this pump was incorporated into the machine, I could deepen my understanding of it.
Still doesn't define how I could increase the power of it, but it's something to go to.