If the leak is explosive and air is rushing out then the valve is in the open position and needs to be pushed in and quarter turned to closed – quickly! I still regularly forget to check that the valve pin is in the ‘closed’ or up position before I start pumping and then experience the agony of having to re-pump the last few psi again to the soundtrack of my paddling friend’s merry derision. It happens!
If the leak is slow then one of two things is happening; Either the two pieces that make up the valve are not screwed together tightly enough, failing to form an airtight seal, or there is dirt, dust or some other object caught in the valve. Do not worry the valves are removable and replaceable.
In the repair kit there is a spanner, a black plastic tool used to tighten the valve up. Deflate your board completely and position the spanner over the valve. Press down with your left hand on the spanner while turning with your right. This action grips the back of the valve in place while the top part tightens down on to the board! It’s a bit like a jam jar and its lid.