I have a mini pump for my bike - I guess for around 3 years now. It's started to give me trouble recently - it's for a presta valve and it screws on, rather than locks on. For some reason when I screw it onto the valve to full tightness the air starts to come out of the tire through the pump. This never happened before. If I unscrew it to around 3/4 tightness it stops but when I pump the tire like this I am not getting good pressure.
Is it broken? Can I fix it?