Hmm, thats interesting Randy. My system3 pumps burped inconsistently as I completed the system3 jugs. I then took my pumps apart and cleaned well with vinegar, installed on Raka jugs, but one (resin) still burps almost every time now. I will try again and look for what you describe.
No big deal, as I use various other measuring methods anyway. But I shouldn't need safety glasses to pump resin! (not the worst idea I guess)

: 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.