Repairing old pneumatics

[Amt0571]

I'm trying to rebuild my old 8837 excavator after 30 years of storage and I noticed that the pump mostly doesn't work. For every 20 pumps, maybe one works and the others don't add any pressure (and in fact leak any pressure already in the circuit).

I also noticed that the big piston leaks on the shaft seal (it retracts, but it doesn't hold pressure), and the metal shaft is blackened from what seems corrosion or residue of a rubber seal. It holds pressure normally in extension though. I tried to open them to see what's happening but felt that if I pushed harder I would damage them.

I checked on bricklink and both parts are available but, being old parts, I fear they'll end up the same. I also checked aliexpress and it seems there are chinese clones available.

Is there a way to repair those parts? or is the best option to buy a working or a cloned part?

Edited December 13, 2024 by Amt0571

[allanp]

There's always the option of buying used working/clone parts but sometimes it's satisfying to bring something back to life yourself, and often cheaper!

There are some things you can do to try to revive them. My process for old used pneumatics is to first try to clean the steel rod, as any imperfection might prevent a good seal. I start with the least aggressive approach using some scotchbrite. You can spin the rod by attaching a lego motor to it, this makes cleaning much faster. If it still looks terrible, or if I want it looking really shiny like new, I have had good results by mounting the rod end to a lego motor to spin it then shining it up nice with metal polish. I have got really bad ones looking brand new but please be careful with this approach (try metal polish as a last resort), metal polishing compounds may have ingredients that can eat away at the top seal. You also definitely want to make sure you get all polishing residue off the steel rod before you retract the piston because metal polishing compounds also contain abrasives. I don't imagine it's good for the seals but if you are careful, apply a tiny amount to the cloth and wipe all the residue off and do what you can to stop it getting into the top seal it does wonders!

After I'm happy that the rod is at least functionally clean I use a thin silicone based lubricant (again, to not eat away at the seals like a petroleum based lubricant might) on the seals by directly lubricating the rod and putting a few drops in the air inlet ports to lubricate the inner seal. 

[Amt0571]

On 12/14/2024 at 10:22 PM, allanp said:

There's always the option of buying used working/clone parts but sometimes it's satisfying to bring something back to life yourself, and often cheaper!

There are some things you can do to try to revive them. My process for old used pneumatics is to first try to clean the steel rod, as any imperfection might prevent a good seal. I start with the least aggressive approach using some scotchbrite. You can spin the rod by attaching a lego motor to it, this makes cleaning much faster. If it still looks terrible, or if I want it looking really shiny like new, I have had good results by mounting the rod end to a lego motor to spin it then shining it up nice with metal polish. I have got really bad ones looking brand new but please be careful with this approach (try metal polish as a last resort), metal polishing compounds may have ingredients that can eat away at the top seal. You also definitely want to make sure you get all polishing residue off the steel rod before you retract the piston because metal polishing compounds also contain abrasives. I don't imagine it's good for the seals but if you are careful, apply a tiny amount to the cloth and wipe all the residue off and do what you can to stop it getting into the top seal it does wonders!

After I'm happy that the rod is at least functionally clean I use a thin silicone based lubricant (again, to not eat away at the seals like a petroleum based lubricant might) on the seals by directly lubricating the rod and putting a few drops in the air inlet ports to lubricate the inner seal. 

Following your instructions I have cleaned the rod and the cylinder is now smoother. It still leaks air around the seal when retracting though, and it doesn't retract unless there's some weight on it, which is not much of a problem as the excavator arm weight seems enough to lower it.

I've been unable to save the pump though. There's no way to make it pump consistently.

In the end, and since they were relatively cheap, I bought a clone cylinder and pump on Aliexpress and also ordered some parts on Bricklink to replace a few missing and broken ones so I can finish building it. I hope the aliexpress parts quality is good enough.

[Mikdun]

When I opened my old pump the rubber gasket on the piston was split in half.

Ended up buying new "compatible" pump, I didn't want to risk buying another old one.

[Amt0571]

2 hours ago, Mikdun said:

When I opened my old pump the rubber gasket on the piston was split in half.

Ended up buying new "compatible" pump, I didn't want to risk buying another old one. 

Yes, that was my thought exactly, as I saw a bunch of them on bricklink that said that they didn't pump in the description, so I suspected they end up failing the same way.

I wanted to build the excavator with original parts, but I have made an exception on the pump and cylinder for this reason.

[Amt0571]

Finally the cylinder and pump replacement arrived. It was not said on the Aliexpress ad, but they turned out to be from "Mould King". They seem to work well and the cylinder has less friction than original lego cylinders which means a slightly smoother movement.

They're visually identical except that both, pump and cylinder, have a "Mould King" logo engraved on the side. It's ugly, but somewhat difficult to see, so it doesn't bother me that much.

[2GodBDGlory]

On 1/9/2025 at 9:46 AM, Amt0571 said:

Finally the cylinder and pump replacement arrived. It was not said on the Aliexpress ad, but they turned out to be from "Mould King". They seem to work well and the cylinder has less friction than original lego cylinders which means a slightly smoother movement.

They're visually identical except that both, pump and cylinder, have a "Mould King" logo engraved on the side. It's ugly, but somewhat difficult to see, so it doesn't bother me that much.

I've got a bunch of pneumatics from Mould King, and in some ways they're really nice (Low friction, and metal shafts on even the smallest cylinders), but there are some other issues. For one, the cylinders seem to stick in one position when the model is sitting, and need a fair bit of pressure or manual movement to get them moving at the start of a play session. For another, the tips tend to pull out of the metal shafts, but that can be fixed with some super glue. I'm quite pleased with their value-for money overall, though!

[Amt0571]

On 1/10/2025 at 6:14 PM, 2GodBDGlory said:

I've got a bunch of pneumatics from Mould King, and in some ways they're really nice (Low friction, and metal shafts on even the smallest cylinders), but there are some other issues. For one, the cylinders seem to stick in one position when the model is sitting, and need a fair bit of pressure or manual movement to get them moving at the start of a play session. For another, the tips tend to pull out of the metal shafts, but that can be fixed with some super glue. I'm quite pleased with their value-for money overall, though!

I have not had the issue of the tips pulling from the shafts yet. I doubt It'll happen considering the use this model will see is mostly as a display.

I have the 42144 material handler and the 42043 Arocs, obviously with original cylinders, and when I pick them up to play the cylinders are usually sticky and I need to move them before playing like you describe with the mould-king ones. The small one that operates the grabber on 42144 and the buckets on 42043 are the worst offenders. I usually move them slightly manually and then they work smoothly.