An air motor for a pump assembly including a drop tube communicating between an upper chamber port and a top plate port and including a longitudinal axis that is at an angle of between about 0° and 10° with respect to each of the upper chamber longitudinal axis and the top plate port longitudinal axis. The drop tube has a substantially constant internal diameter, a first generally bulbous end, a second generally bulbous end, and first and second slots defined in the respective first and second bulbous ends. First and second seals are positioned in the respective first and second slots, and the first and second seals air-tightly seal an outer surface of the drop tube within the upper chamber port and the top plate port.