How can I build a pneumatic cylinder out of PVC piping?

Use a rubber gasket from an espresso machine around the plunger. Good luck!

It is my understanding that you are planning to build a single acting cylinder for the purpose.I suggest you build a tie-rod type cylinder. I did a back of the envelope calculation and with the size of cylinder you are going to use the load you are lifting is quite small.The challenge will be obtaining gas seals between the plunger and cylinder while it is traveling. End Caps: Get two end plates machined with an appropriate number of holes(four should do). Mill a circular slot into the end cap to accommodate the end of the PVC pipe(with some extra area to accommodate the sealing-more on that below). Sealing the end caps: You might find it difficult to obtain good O-rings for the  purpose,assembling them also might take time hence I suggest you to use  Poly-Urethane to obtain gas seals at the ends it comes as two liquids  which you can blend to obtain the foam.Since the pressures you are going  to operate at would not be high that should be fine. The  orange color you see at the ends are gaskets formed using PU,you would  need to build a make shift mold for the purpose. In case you find good  O-rings then that's okay too. Dress the ends of the Pipe: PVC pipes are generally shipped with sawed off ends,dress the ends and if possible use some sheet metal to make a liner around the edges as it could break or crack when you compress them. Plunger: The inside surface of the pipe is anything but uniform and during shipping creep causes the pipes to deform along its axis.Try to obtain as straight a pipe as possible.I feel you will need to get a piston machined in steel and cut at least two groves along its axis at a few millimeter intervals to house a gas seal(if you cant find a good one you could use PU for this purpose as well,however you will need to boost the hardener content in the mixture to increase its shore hardness,either ways using PU is not going to give you a lot of service time.But the good news is with your tie rod type build it will be easy to change the seals.Perhaps you should have a spare piston. Tie Rods: You should be able to get tie rods long enough from building supplies contractors,I have seen some really long studs used for HVAC duct installation etc. In the end it should look more like this,with a plunger rod sticking out from one side. Direction control: To control the up and down movement of the lift you need to fashion a crude D.C valve.Just add a check valve(non-return valve in some countries) in the inlet circuit to prevent the air from rushing back from the cylinder through the inlet.Add another port to release the inlet air when you want to bring the cylinder back.Make sure you add a pressure relief valve set at about 3bar in the inlet circuit, just in case;I think that is also required by the law depends where you are from though. You could improve the system a lot in other ways as well,like adding dampening to the cylinder to prevent the cylinder from crashing back when the pressure is released,but this is like the bare essentials. Pictures courtesy:  http://hydraulicspneumatics.com/other-technologies/chapter-15-fluid-power-actuators-part-1 http://www.michiganmechanical.com/cylinder_repair.html http://www.diytrade.com/china/pd/6359400/PVC_Pipe_Fitting_Compression_Coupling.html