Is it normal for a presta valve to unscrew into two pieces? I have a Continental Road inner tube with a threaded presta valve. I also have a threaded bike pump which I screw onto the valve to inflate the tube.

When I try to unscrew the pump, the uppermost part of the valve unscrews and detaches from the main body of the valve and all the air rushes out.



Is it normal for a presta valve to be separable into two parts like this? How can I prevent this from happening?
The correct answer is yes and no. There are in fact two types of Presta valves: One where the core can be removed. Those are often used with tubular tyres so that the inner tube can be filled with a sealant (latex) fluid. Some higher quality tubes also have removable core valves for the same purpose. Normal and cheaper tubes have Presta valves where the core cannot be removed.

There is a special tool to be used for handling the core although like @Michael said the (careful) use of pliers also works.

Another reason for a removable core is the possibility to use valve extensions with high rims.