I ran into the burping pump problem in the middle of laying up a carbon fibre rudder. You can bet I said a few choice words. I muddled through and the rudder came out fine.

Afterwards I disassembled the pumps and found the problem. The epoxy had gotten quite gummy around the steel balls that create check-valves at the bottom and in the head of the pump. Consequently they were not functioning properly and caused the burp.

One hour and 1/2 a gallon of lacquer thinner later they're shiny clean and pumping normally again. From now on I'll make it a point to clean the pumps before any major epoxy work.