Hi,

Yes there's a Schrader valve inside the pressostat fitting. Similar to the tyre valve only that its sealing rubber is R134a compatible. Normally you're able screw out the pressostat and replace it with a new one without any incident. But there are cases of the valve got stuck in the "pressed" open position for so long by the pressostat and didn't seal back when pressostat removed.

You could screw the pressostat half-way out then screw it in again a few times. Hopefully this would "awaken" the valve to work. But still expect the unexpected to happen.

In case the above didn't work you'll know it soon enough. You could screw in back the pressostat quickly to stop the refrigerant from spewing out. You'll then have to evacuate (empty) the system thru the usual method, replacing the pressostat only after that.

I'm not familiar with that NAPA part. IME better to use OE Volvo or at least a part from reputable manufacturer such as 4-seasons. I ended up ordering OE Volvo to replace a Santech pressostat last time.

Regards,
Amarin.