TN93 figure 17

Figure 17. Assembled versions of the Bell (left), Air Master (middle), and Tesco (right) vacuum pumps.

To check your pump, with the bottom/gasket side of the PVC cap on the palm of your hand, draw up on the pump handle. As you operate the pump, you should feel the PVC cap draw up tightly against your hand. If the check valve is working properly, the PVC cap should still stick to your hand even on a down stroke, or when you stop operating the pump. The check valve works by gravity, so it will function best when a vacuum is drawn with the tire stem oriented vertically.