Gas Cap Question

This is kind of a side issue, but I believe gas caps have to let vent air into the tank when there is suction but prevent fumes from exiting through the cap. I was able to test the aftermarket cap I was using and it took about 2 psi of vacuum for it to allow suction on it to relieve. 2 psi of suction pressure on a plastic gas tank the size of a Tahoe's amounts to a lot of force compressing it. Information on allowable suction to open the air vent on a gas cap is hard to find but I did find some info about Ford caps and the maximum suction allowable by spec to open their cap vent was something like less than 1/2 psi. So I ordered an OEM expensive cap. When tested, the amount of suction to open that cap vent was so low it was not measurable on my hand suction pump, and certainly less than 1/2 psi, so buying a cheap cap could cause potentially problems.