I own a LEGO Technic Unimog U-400 (part number 8110) and have been very pleased so far with the performance of its pneumatic system. However, after storing for just a month or two (after a good 5+ months of hard use), the pneumatic actuators in the crane did not work, even with brand new batteries. I disconnected all of the hoses and felt that the actuators had more friction than they had previously had when the model was working well, which led me to oil them with vegetable oil, along with the pneumatic piston pump.

When I reconnected all of the hoses, switches, connections and tried it out, the second actuator (the one controlled by the middle switch) and the third actuator (the one that powers the claw) still didn't work, while the first actuator (the one controlled by the first switch from the left) worked as good as new. I have checked and double checked the connections. What is the problem here and how can I fix it? Apologies for bad grammar and/or bad formatting.

EDIT: I tested several times and the first cylinder actually isn't working well either. So really none of the three is working now.

  • 1
    I fear that after "oiled it with vegetable oil" you've ruled out help from Lego, and you've done something few of us have ever done. Typically vegetable oil oxidises and turns sticky when exposed to air, so you may need to clean that off and use silicon grease like the stuff Lego uses.
    – Móż
    Nov 12, 2015 at 21:47
  • Thanks, wish I had looked into vegetable oil in the first place. That crud's probably going to be really hard to get off.
    – CC_22224
    Jan 19, 2016 at 0:58

1 Answer 1


I was going to recommend lubricating the pistons, but seeing you already did exactly that, I can only say you should try to isolate the source(s) of the problem.

Since you have at least one working cylinder, the compressor and the feed tubes should be all right. This leaves the following possible points of failure: switches, subsystem hoses, working cylinders. Try disconnecting the hoses from the switches to see if the valves work well, and try connecting the compressor directly to the hoses the valves control to check the individual pneumatic circuits.

If a hose has developed a leak, you can also find it using a long hose (to protect the electronics) that is known to be leakproof and a bowl of water.

If the pistons are proven to be stuck, you can try the steps outlined in this answer.

  • I just submerged the entirety of the lower part of the Unimog's pneumatic crane (all switches and many of the connections), then powered it up and tried it out. I did the same with every part of the pneumatic system, and found that there was no air leakage anywhere. I am thinking that the problem could be deterioration with the seals inside the cylinders, but this seems unlikely given the storage conditions: dry, 60-70 degrees, no sunlight, no exposure to chemicals or other contaminants- and the fact that all three cylinders failed rather than just one or two.
    – CC_22224
    Jul 15, 2015 at 14:42
  • There is still the possibility of either stuck cylinders or deteriorated seals. Can you feel or otherwise sense air escaping the cylinders? If not, then they seal well, but might be stuck. In this case can you move them by hand? If they can move under pressure and while being assisted, then they might need just a few (or a lot) of these assisted cycles to even the lubrication inside.
    – zovits
    Jul 15, 2015 at 16:01
  • I have checked all of the hoses and connections, there is no leakage from the actuators, hoses or any of the connections. I have actuated them by hand many times to even the lubricant (olive oil), but no dice. the one small actuator on the claw is especially sticky. I can think of no conceivable answer to why they should be acting like this, and its driving me nuts. Please Help!
    – CC_22224
    Aug 1, 2015 at 22:17

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