Abstract: A gas venting mechanism includes a valve chamber that is at least partially filled with a liquid. At the top of the chamber, an outlet valve allows the gas to escape but retains the liquid. Air is admitted to the chamber through an inlet valve that prevents backflow of the liquid into a passage by which the gas is supplied. The chamber is transparent so that the venting of gas can be verified by observation. A piston that reduces the effective volume of the chamber facilitates testing of the chamber for leakage of the liquid. The venting mechanism can be advantageously combined with a degasser in an aircraft hydraulic system.