When do you know if you need a new O Ring? Normally you will notice that your pump is not pushing out much or any air. This can be caused by a stretching of the O Ring. To diagnose correctly just follow this simple test. Stand the pump on flat ground. Discconnect the pump hose. Then pull the handle all the way up so it is at it's fullest extension. Next, place the palm of one hand over the outlet hole (where the hose screws to) and push against it so it creates a seal. Then with the other hand push down on the pump handle as if pumping. If you can push the handle all the way down without any resistance then you need a new O Ring.
How do you change the O Ring? It's easy. All you need is a screwdriver. Use the screwdriver to remove the 4 silver screws that are holding the black or grey "cap" on to the top of the pump chambers. Then pull the pump shaft(s) all the way out of the pump. At the end of the pump shaft(s) you will see a white disc with a black O Ring around it. Just pull this off and swap it with a new one. Try and perserve as much of the grease as you can. If you want to add more grease then choose Lithimum Grease. This can be purchased from most cycling shops.
