If air gets inside the valve, it is ruined.

The controlled flow of electrons inside the valve requires a vacuum. If the seal is breached and air gets inside, the valve will no longer work at all, and in fact if you power it up you may even get fireworks as the oxygen inside allows internal components to burn.

Fortunately there’s a very easy way to tell if a valve has air inside it. Take a look at the two examples below:

The valve on the right is in good condition. Note the mirrored sliver coating on the inside of the glass at the top. All valves have this, usually at the top of the tube but sometimes on the side.

In the one on the left, the insides have been exposed to air. The mirror coating has turned a powdery white. 

Usually this will happen if the glass is broken or cracked, but occasionally there can be a manufacturing defect where the pins at the bottom are not sealed properly, and the repeated expansion and contraction with heating and cooling, will open up a small gap, enough for air to get in.

Once this has happened, you can’t somehow suck the air out again and re-seal it.