Bobcat Boom Leaks Down

[Novanut 63]

I have a 2004 bobcat mini excavator that the boom leaks down. We have been trying to bring it back from a hard life. We have had all the cylinders rebuilt, new lines and swivel valve was rebuilt as well. New hydraulic filters and oil. Digs fine but the boom will drop while you are moving, and throttle is at max.
Any help would be appreciated.

 

[ROUSTABOUT]

Re pack the cylinder.

 

[Seven Gables]

I have a 2004 bobcat mini excavator that the boom leaks down. We have been trying to bring it back from a hard life. We have had all the cylinders rebuilt, new lines and swivel valve was rebuilt as well. New hydraulic filters and oil. Digs fine but the boom will drop while you are moving, and throttle is at max.
Any help would be appreciated.

Does it drop if you’re not moving and at idle?. I’d think the boom cylinder would be hydraulically locked on the down/retract stroke with its control valve in neutral. So I’d be inclined to suspect the valve is at fault, spool hanging up or internal wear.

 

[rScotty]

If the cylinder leaks down in compression & without external leaks then it's more likely worn control valve rather than cylinder seals.

 

[5030]

Usually the control valving and it's usually a small bit of dirt under the seal mounting surface or worn and hardened seals.

I have two basically identical Kubota M series tractors and one leaks down (FEL) and the other don't. Not a big deal for me so I ignore it.

 

[oldnslo]

Like rscotty states for the boom to leak down without external leaks the valve must leak. Since this is older machine have you checked the linkage to the direction valve to make sure that it is not binding. Another thing to check is the spring assembly on the valve section. Is it full of crud not allowing the spool to return to home position properly. There are no seals on the directional valve that would allow the cylinder to drift.

 

[ROUSTABOUT]

The piston can leak without any external sign. Plus it's inexpensive, so you can eliminate as a culprit.

 

[oldnslo]

The piston can leak without any external sign. Plus it's inexpensive, so you can eliminate as a culprit.

Cylinder will not drift in retract mode without an external leak.

 

[ROUSTABOUT]

Cylinder will not drift in retract mode without an external leak.

So you're telling me if I take a cylinder apart and remove all the seals off the piston, put cylinder back together, it won't move? I've taken them apart and the rubber came out as crumbs. But you tell me it won't move?

 

[Seven Gables]

So you're telling me if I take a cylinder apart and remove all the seals off the piston, put cylinder back together, it won't move? I've taken them apart and the rubber came out as crumbs. But you tell me it won't move?

No it won’t retract in if it’s filled with oil on both sides of the piston even without seals , there’s no room for the rod. It will extend out with bad piston seals cause the oil has room to go from the small volume side to large volume side