On your baby steps towards medium/high vacuum, you will be tempted to go cheap on some things. Tubes and connectors are fine, there is not much that can go wrong with those, however, there is a point that will bring you failure real soon.
You will probably have started with a refrigeration vacuum pump (we all do, don’t be ashamed) and it’s interesting, but headachy 1/4SAE connectors. For those, there are not that much range in valves to choose, and the most common is this one:
For sure there should be high quality ones out there, but if you have the money for those, better skip this step and go with KF flanges and a good pump.
For the ones trapped in the 1/4SAE world, be careful. If you must use one of these valves, they WILL NOT seal the same way from one side than the other.
Here’s a hand drawn schematic of the insides:
As you can see, the only confiable seal is the rightside. the rubber gasket is under pressure from the ball and the body, so, if any of those seals has to be any good, this one will be. The leftside seals are not especially bad, nor extremely good, they will do, up to a point.
However, the ball actuator o-ring, sitting between two metal plates is something to be aware of. Altough the ball axle itself is polished and nice, the o-ring seat in the body will leave much to be desired, thus, making a less than improbable good seal. Should you have your vacuum chamber connected to the left (vacuum pump to the right) of this valve, I assure you it’s vacuum will not last, both open and closed valve.
Having it the other way around will not prevent circuit leaks (keeping you from the ultimate vacuum your cheap pump could attain) but at least with the valve closed, your chamber stands a chance.