Pumps tend to burp when the epoxy is cool and viscous, when you are trying to pump too fast or when they are near empty, or some combination of the above. Because I have a cool workshop, I have a little metal cabinet and keep my epoxy at 75 degrees. With a slow steady push I never have problem with WEST---the epoxy I use most. Also, on older pumps the ball valves sometimes hang up and cause problems, but heating the epoxy to 100 degrees or so dissolves the crud and they then work fine. If you do get a burp, such set it aside and run a couple of more hits through until you get an entire charge. Just pour the partial hit back into the can. Because I am paranoid, from time to time I calibrate the amount of epoxy per stroke by weighing it.