John,
They replaced the cylinders just on Rolling Thunders set. They still sell the air up/spring down units. I like the idea of the spring extending the retracts if you happen to loss air pressure. Seems to me they could combine the systems so they have air up/ spring assisted air down. Would be the same as air up/air down cylinder except with a spring in the cylinder that pushes on the actuator arm in the down direction in case air pressure becomes to low. Kind of had to explain. Seems to me it would be simple to put the right size spring in the cylinder on the side that pushes the actuator arm down.
Ralph White