Testing of Valves

You have vacuum? Which one - meaning how much vacuum? Manufacturers also specify also how much of a vacuum a valve is vacuum sustaining. You can test this also when you have vacuum and a vacuum meter and test it (even on both sides). Connect your valve to a vacuum tester (type automobile pump) and see how the needle of the meter acts over time. If you are talking about high vacuum, get yourself some better measuring stuff. If your valve is cheap and it works, lets us know the make and model. We might be interested too.

The manufacturer's data sheet for each valve says it all.

Close one end using O-ring and and other end of valve on vacuum tester apply vacumm and stop vacuum tester and monitor pressure drop with time

Compare that with flat glass sheet vacuum stability. We do this as routine and will work for you also

Vacuum is the absence of air or atmospheric pressure. I believe maximum negative pressure possible or achievable is about 760 mmHg. Noticeable differences will only depend on the rate or how fast this negative pressure is created. Any positive pressure air /liquid valve rated to exceed or sustain an equivalent positive pressure should work as well in negative pressure applications.