Dual Valve Pump Head - How Does It Work? Whats Inside?

I disassemble a dual valve bike pump head and explain how it works, including why it doesn't blow air out the open hole. I also give a tip for extending the life of a leaking dual valve pump head, then reassemble it. The pump is a Park Tool PFP-3. On this model, the lever is pulled up to lock onto the valve stem; on other models, the lever is up when put on the valve stem and pushed down to lock.