The pump has a barrel with vent ports located therein and a plunger that reciprocates inside the barrel. The barrel has a standing valve and the plunger has a traveling valve. The plunger has a first portion with a seal between the plunger and the barrel and a second portion with a clearance between the plunger and the barrel. On the downstroke, gas contained in the compression chamber between the two valves vents through the clearance and out of the barrel through the vent port into the well bore. When the plunger contacts liquid in the compression chamber, the liquid enters the clearance and forms a seal, wherein the plunger traveling valve opens. The plunger can also be equipped with a second traveling valve and a vent in the plunger between the two traveling valves in order to equalize pressure across the first traveling valve at the bottom of the downstroke. This enables the standing valve to open much faster on the upstroke.