sounds like bad schrader valve
It really might just be that your engine was too hot. If you try to take it off under high temperatures, the built up heat pressure can make that sound. Take it off all the way, and it has been known to shoot all over people and give them severe burns. What I would do is try taking it off at a time when the car has not been driven for a few hours.Also, during driving be sure to occasionally check your temperature gauge. If you are overheating, it will definitely do this, and on top of that, it will total your vehicle.
Now one way overheating will be caused is by a lack of coolant. It can also be from a broken fan or dysfunctional part used to cool the vehicle. I have seen this first had. My brother's fan was not properly working, and just as soon as we pulled over to check, the same thing happened. A hissing sound occurred (followed by a smoke).
Again, make sure your vehicle has some time to cool down before attempting to take off the cap. There is pressure build-up, just as you suspected. I'd say try taking it off at a time that it hasn't been driven for a few hours. If it still does the sound, I don't know. That's weird. lol.
The cap on the A/C line of a car is a lot like the air valve on a bicycle tire. The cap just protects the valve underneath. There should not be any pressure on the cap when it is removed. So, yes, I would agree with the guy. I also recommend that you stay far, far away from adding your own refrigerant. The components in the A/C system are very sensitive to pressure changes. Adding just a little too much can do hundreds of dollars of damage to the system. If you don't connect the valve properly, you can also make a huge mess. If you aren't careful when connecting and disconnecting, the refrigerant can spray out of the can...on you, in your face, and in your mouth and nose. It's not something you want to be breathing in. If you need your A/C, take it to someone who can do it right and give you a warranty on the work.
Let me be clear. If you know what you are doing, it might be worth trying to do yourself. The fact that you had to ask where to add it and don't know about the valve tells me that you may not be adequately prepared to do this job properly or safely.
I was at advance Auto parts the other day and I asked the man to show me where I could add refrigerant. When we twisted the cap it made a noise as if pressure were built up, like when you open a can of pop, he said that it shouldn't do that? Is this true? It's a 14 year old Lincoln Continental, I didn't figure it was an issue, I expected it to be normal
