However, anyone who has followed the Technic line for a time, will know that it is not the second Pneumatic System installment, it is in fact the fourth. See the four versions of the basic cylinder below:
With the observation that the Pneumatic System is often used to lift things, e.g., the boom of a construction vehicle, it is interesting to see this in a historic context. Here is how lifting has evolved in the Technic line:
1977: Rack and pinion
Technic itself was brand new in 1977, and the way to transform a rotational force into a translation (lifting) back then, was by using a rack and pinion:
This was used, e.g., in the 950 Forklift.
The downside of this method, is that the construction becomes quite fragile, and it is very complicated to get the geometry right. For a child to build a well functioning mechanism based on this principle is very hard, and can be demotivating.
To get the action more easily available, the first pneumatic system was soon introduced.
1984: Pneumatic System (first version)
In the first pneumatic system, used in, e.g., 8040 Universal Set, the cylinder was single acting, i.e., it had only one connection for the air hoses:
This required the special 2x4 valve pictured above, which turned the blowing force from the pump (left) into both blowing and suction, as decided with the toggle valve. It worked well, and made it much easier for a child to make powerful lifting booms and similar constructions.
As long as you can get an air hose out to the cylinder, you can place it pretty much anywhere you want on a model, which makes it very flexible and fun to build with.
1989: Pneumatic System (second version)
Already after five years, the pneumatic system was superseded with a second version. Now, the cylinders became double acting, i.e., they had two air hose connections each. This means that the special valve which creates the suction air stream is no longer needed, but the downside is that you need to connect one more hose out to each cylinder.
It was used in, e.g., the very big and complex 8862 Backhoe Grader.
A smaller cylinder, and also a mini-pump were later introduced, as this system lived for a long period. The mini-pump had an odd geometry, making it hard to use effectively in a model: It was not an exact stud unit length neither at the shortest, nor the longest extension.
1991: Flex System
The flex-system was introduced in the 8856 Whirlwind Rescue helicopter, as an alternative way to transfer energy from one place to another in a model.
As the name suggests, it is quite flexible, however, you must have a hose and a "wire" with exactly the length you need to fit when making your own model, which can be frustrating. So this system works well with some official sets, where you get exactly the right lengths, but it is not so easy to use in your own creations.
1995: Worm Gear and Liftarm
The worm gear appeared already in 1990, but it was not until 1995 that it was used for lifting. The special yellow gear box below made it easy to create a boom like creation:
This also introduced the liftarms, a harbinger of the soon to come studless Technic system. They are often called liftarms, because they use the rotational force from an axle to lift something connected to the other end. An example 1x4 liftarm can be seen in grey above.
It was used in, e.g., the 8280 Fire Engine. Even if the gear box is not commonly seen today, the same technique has been used in countless Technic sets over the years, and is still widely used these days.
Personally, I'm not a big fan of this construction technique. I don't think it is realistic: This it not nearly like how real machines work. Hence, I think it is not in line with the "As In Reality" Technic slogan.
2008: Linear Actuator (61927)
Used in, e.g., the 8295 Telescopic Handler, the Linear Actuators became an alternative to the Pneumatic System. Like the Pneumatic System, this also looks like real life hydraulic cylinders, in line with the "As In Reality" slogan:
Just like the original rack and pinion systems, however, there is the problem of getting the rotational energy out to the cylinders in the first place. This can be tricky, even if LEGO does supply some specially made brackets for this purpose:
The bracket for connecting a bevel gear (top), is the most common, and is in, e.g., 42043 Mercedes-Benz Arocs. The bracket for connecting a motor (bottom), is very rare, and has only appeared in two sets. Most recently, the 8264 Hauler in 2009.
Take a look at the 8043 Motorized Excavator for an example where axles bring the rotational energy out to a three segment excavator boom. This is immensely complicated. Building something similar in your own creation is hard even for an adult.
Looking at the years in the timeline, it can appear that there was not a lot of innovation in LEGO Technic around the turn of the century. It is common to say that this was a low period for LEGO in general. However, keep in mind that this was the time when the Mindstroms sets were developed, so in fairness there was certainly innovation in the Technic line.
2011: Small Linear Actuator (92693)
In the 8069 Backhoe Loader, a smaller linear actuator was introduced.
2011: Pneumatic System (third version)
Just when people thought the linear actuators had replaced the pneumatic system, a third version is included in the licenced 8110 Mercedes Benz Unimog:
This systems marks the transition to the studless building technique, as it is no longer possible to connect the cylinder to a traditional brick.
Also, the mini-pump now has a more sensible geometry, making it much easier to build into your own creation. There is a two stud unit length between minimum and maximum extension, so you can power it with a rotating 3x3 wheel.
2015: Pneumatic System V2 (forth version)
Finally, with the LEGO Technic 42053 Volvo EW160E, we get a new revision of the Pneumatic System:
This takes the final step into the studless system, in my opinion, as the width of the connection is now only one stud, not two as before (new cylinder to the left below):
Another innovation, is that the air hose connection has a narrow tap, making it easier to attach a hose (new to the left below):
Other than these changes, the new system is much like the previous installment. There is now a better colour coding: Blue cylinders are pumps, yellow cylinders do the action.
I think the reintroduction of the pneumatic system makes it clear that LEGO understands the challenges with the competing linear actuators: They are very complex to use in your own creations. The pneumatic cylinders, on the other hand, can be placed pretty much anywhere on a model, and you don't need to worry about getting a gear train out to power it.
Using pneumatic cylinders, it is easy to create models with powerful action quickly, even for children. Which is what LEGO Technic is all about, anyway.