cat 236 - i just rebuilt lift cylinders, but lift arms keeps leaking down

The thing with standard hydraulic cylinders is if the piston seals are gone, they will only drift out, not in.

92U 3406 said:

The thing with standard hydraulic cylinders is if the piston seals are gone, they will only drift out, not in.

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Depends on the rest of the system and the load imposed. When they go into displacement mode, with only the area of the rod to hold the load, pressures can shoot up increasing the typically minor leakage in the valves, or in some cases opening the port reliefs.

lantraxco said:

Depends on the rest of the system and the load imposed. When they go into displacement mode, with only the area of the rod to hold the load, pressures can shoot up increasing the typically minor leakage in the valves, or in some cases opening the port reliefs.

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Definitely. I meant just a sole, isolated cylinder should not drift inwards.

I took apart the lift cylinders again to take measurements with calipers and bore gauge.

piston rod shaft diameter = 40mm (39.96mm)
piston head seal diameter = 60.55mm (largest diameter seal on the piston head)
piston head wear rings = 59.90mm
cylinder bore = 60mm + 1/1000inch

there isn't much force required to pull the piston out of the bore.
I can pull the piston out with one hand, and push it back into the cylinder with one hand (50lbs of force required maybe)
These clearances are probably too loose, and is making the lift cylinder operate in displacement mode?

It doesn't seem like the seals are the problem in my opinion.

Was the boom lowering valve checked?
Were the boom UP and Down relief cartridges swapped?

I'm lurking

muddog1975 said:

If you still think its in the cylinders, (bypassing internally) raise your boom up safely hold the boom up in the air. Release your hydraulic pressure from the machine( key on no start...rotate and move joysticks) to release pressure. Remove your lift cylinders hyd hoses ( both) and cap both of them off with metal caps. Take a permanent marker and mark your rod. Then releases the boom from whatever you where holding it up with. Watch to see if the boom drops down. If it does its in your cylinders...if not you know its bypassing some where else. Possible control valve spool...etc. good luck... let us know what you find.

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This would work if the lift cylinders dropped without any weight on the bucket, and it seems its only dropping with a load. but yes it would work, I know cause I did this with a backhoe, but the lift cylinders where dropping without a load, turns out when capped the boom did not drop. spent more time troubleshooting and turns out the spool in the lift valve was damaged with a big deep scratch.
I guess in this situation if the same was applied, but with a load on the bucket youll get a good test to verify if it is in fact the cylinders or not. I havent seen the machine but I dont think it is the cylinders. Its something else.