1984 EU Technic catalog
The first 3 pneumatic sets, including the 8851 Excavator and 8040 Pneumatic Universal Set as well as the 2nd Idea Book were released in Europe. In addition to the pneumatic elements, the threaded cross axles were new for 1984.
The pneumatics: The first pneumatic system included a pneumatic pump, cylinder, switch, and a distribution block, which were connected as a circuit with hollow rubber tubing. The original pneumatic pump and cylinder were single acting: pushing down on the pump created pressure in the system and pulling up on the pump created suction on the system.
1984 pneumatics
Because the original system was closed, Lego designed the piston head in the pump and cylinder to act as the pressure release valve. As described in the 1984 patent application, the piston was constructed from a flexible material that would normally contact the walls and seal the cylinder, when the pressure in the working chamber became too high the head would deform, breaking the seal, and allow the pressure to escape.
UK patent 2,139,318 A
8843 Forklift:
The 2nd forklift and first pneumatic model. The model's construction was typical of mid 1980's sets. It relied heavily on Technic beams, connectors and rounded plates rather than standard bricks. The model featured rack and pinion steering controlled by a 2x2 round brick on the top and movable forks controlled by a chain and pneumatic piston.

1984 sets:

8020 Expert Builder Set
8040 Pneumatic Expert Builder Set
8843 Forklift
8851 Excavator
8889 Idea Book
By Isodomus