jwiger

I'm coming in late to this conversation, have you studied this system at all? http://en.wikipedia.org/wiki/Walschaerts_valve_gear#Purpose

I'm curious how this responds when the torque on the first output suddenly drops off. Does it revert back to the first stage, or stay on the second stage?

A differential on a trailer axle is a waste of time. Typically trailer wheels are mounted on spindles or bearing similar to the front wheels of a rear wheel drive vehicle. These tires are scrubing because they are rotating on a comon axle shaft. Put them on individual shafts and you will not see this problem. There are several ways to handle heavy loads used here in the US. Most use fixed axles and simply live with the sliding of tires side to side as the trailer is turned. Typically on the highway the turns are gentle that the flex of the tire permits this with no bad effects. Turning tighter turns the sliding will be very noticeable. Some vehicles use axles like the ones you put on your trailer, they are some times refered to passive steer or drag axles. Trucks that deliever concrete often use these in the middle of the chassis and also at the back. In both cases the tires(axles) are lowered to drive on public roads to distribute the weight and not damage the road surface. When the vehicles need to make tight turns they are raised up to reduce turning resistance. You may find it easier to use a system like this -with out trying to steer the axles.

If you simply want to test for leaks i would suggest running an axle through both ends of a cylinder, then one or two sturdy rubber bands around the axles. hook the cylinder up directly to a pump and pump it just enough to fully extend it. If it stays extended then the pump and cylinder are good enough to use. If the rubber bands make the cylinder retract than you have a leak in your pump and/or you cylinder. You can trouble shoot this by replacing the pump or the cylinder, until you find a good combination. Once the pumps and cylinders pass that test you can test the retracting of cylinders using LEGO shock absorbers on the axles. If the cylinders stay still for ten to twenty minutes then they are good enough to use in models. Just be sure to use just enough air pressure to fully extend and retract them, don't keep pumping once they max out. Valves can be tested by simply adding them in-line between the pumps and cylinders and repeating the tests.