My 200 Ford Ranger has been showing signs of having a low level of refrigerant in the A/C system. As I started removing the low pressure valve cap I heard a hissing sound like air coming out through the valve itself. Before I got the cap completely off the valve, I changed my mind and twisted it back on.

I never add refrigerant to an A/C system before, so I was sort of confused with what I noticed. I thought that valve was a schrader type valve which would keep air from coming out of the system unless you used a connector/hose like the ones you get with the refrigerant can.

Am I wrong about that?

Yes, it is a type of Schrader valve in there.

Sometimes a little bit of refrigerant leaks around it and is captured by the sealing cap. If it hisses for a second and stops, that's OK.

If it kept on hissing tho as you were unscrewing, the valve is bad. If you take it in, they can change the valve without evacuating the system by using special tools.