A couple of ball check valves are used for air input and output. The resulting pump isn’t the prettiest of pumps, and it could probably do with a bit of balancing as it rattles somewhat.

BESTOOL Air actuated hydraulic treadle pump internal ... Air and oil inlet filters for reduced risk of damage from contamination. Air powered hydraulic pumps maximum adjustable pressure: 10,000PSI ...

Gear pumps are the most common type of positive displacement pump used. Typically, a rotating assembly of two gears (a drive gear and an idler) moves to create suction at the pump inlet and draw in ...