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Some of my pneumatics parts, mostly pistons, have seals which no longer prevent the air from escaping completely. On some parts they are unreliable and on others they are not working at all.

Why do those parts become leaky, how can I minimize the risk of it happening with maintenance and/or proper storage, and is there any way to repair already leaky parts?


Zhaph suggested disassembling the piston and replacing the seal with an o-ring, however the seal in question was not a simple o-ring and getting a replacement is properly difficult: Parts which makes up a Lego piston From the left:

  1. Top piece locking everything together
  2. Top rubber seal, with a bit particular shape
  3. Bottom rubber seal, which is just a wide rubber ring
  4. A plastic spacer between the two rubber seals

While the top piece appears to be safe to remove, the bottom rubber seal easily gets damaged when you try to force it off the piston. The amount of pressure it could retain after forcing if off a couple a times became limited.

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I hadn't realized what the going rate for pneumatics was on bricklink. –  retracile Dec 3 '11 at 21:13
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Until recently no sets containing pneumatics had been produced for years, so they had become rather rare while still having demand. I thought it would have improved by now, but I guess stock is still low... –  Sebastian Wahl Dec 3 '11 at 22:49
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1 Answer

The parts become leaky due to the seals inside the pistons deteriorating.

Typically, the seal is made with an "o-ring" - a circular rubber band (cross section as well as the shape) - if this is the case in the LEGO pistons, and then it's possible the o-rings are made with similar rubber to the LEGO rubber bands, and then they will deteriorate over time in the same way the rubber bands do.

I've not tried dismantling any of my pistons, so I couldn't say how easy they are to repair - they look like you should be able to unclip the black part and access the piston that way (there's a YouTube clip of someone doing so here, you can just see the piston head too, but I can't confirm the o-ring).

A good car spares or similar type of shop should be able to sell a selection of replacements.

From my time spent diving, there's not much you can do to stop this happening over time, other than keeping an eye on them and checking them regularly for replacement.

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I have tried disassembling one and there are three rings. In the bottom of the piston is a fairly wide ring made of rubber. Then comes a small spacer of ordinary plastic. At the top there is a fairly complex part made of rubber and sadly this is the one which is normally causing trouble. Getting a replacement for this one will be difficult... I will add a photo in the question. –  Sebastian Wahl Dec 3 '11 at 22:46
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