If it is 1.25 gallon above liquid at a nominal 15 psig (half an atmosphere that gives you five pints if adjusted to normal pressure air). This is enough to cause some oxidation.
A theory of operation:
If I was trying to minimize this I would look for a way to put an inert gas in the headspace. The easiest gas is CO2 because you can minimize degassing in which case it naturally comes out and brings the headspace back to ambient pressure, ,,, however with 25% of normal ppm oxygen (equal to five cups if normal air) Pumping out to half an atmosphere again removes more of the oxygen and would bring it down to 12.5% of a normal atmosphere (2.5 cups if normal air). Another two cycles of pumping out followed by degassing the wine would get you to an oxygen equal to five ounces of head space which is tolerable. Better yet pull it down a sixth time to get to oxygen equal to 2.5 ounces of headspace. At this level time shouldn’t be an issue.
First of all CO2 is your friend! Make it work for you/ ie don’t waste it. Next, The check valve in a headspace eliminator is a good safety and what I have seen says you can trust them to be one way. A vacuum gauge will let you know what you have pulled and can be on the pump. Expect that there will be some oxygen diffusion on a cork set up BUT it will be negligible as long as the inside and outside are at a similar pressure.
