 Sometimes, a metering or similar pump has to be primed before operation, i. e. the pump head filled with the liquid to be pumped.When gas bubbles enter a pump head, the compression motion compresses the gas but has a hard time forcing it out of the pump head.The pump may stop pumping liquid with gas bubbles in the pump head even though mechanically the pump is going through the motions, repeatedly compressing and decompressing the bubbles.