I have always been fascinated by the Lego Pneumatics. In this model I experimented with suction rather than pressure. This model is only the pump itself. Unfortunatly I am not able to find a special pneumatic part in LDraw, for the illustrations (haven't figured out how to draw tubes either). It's an old valve piece that came with the first pneumatic sets, a 2x4 Brick with three tube-holders coming out. The one-way valve seperates suction and pressure, which helps me build up suction in the airtank. Also, the pumps I am using are from the old pneumatic models, as the newer once has a valve, taking away the suction. (Thank you Brian, for finding a long forgotten set in your closet!) I built in a suction measure device to help the RCX keep the suction level stable. It uses a Light sensor and the old pneumatic pump. Attached to the pump is three levels of grayshade and as the suction builds the pump will slowly start to contract, and the Light sensor picks up the level. (See picture below)

I have added a 2 min video of my first draft model, showing the full functionality with a suctioncup.
Click here. (Size 3,2Mb - MPEG4 format - 2 min)

Download the LDraw file here. (SuctionPump.dat - 5Kb - Right-click if necessary)