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

 

[oldnslo]

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?

If a single rod cylinder is full of oil it can not drift in the retract position without an external leak. The reason for this is the rod takes up volume so oil can not move from cap end to rod end.

Yes bad seals and a leaking valve can contribute to drift. On initial post the OP stated cylinders were rebuilt.

 

[4570Man]

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?

If you take the cylinder apart and remove the piston seals or removed the whole piston for that matter then filled the barrel completely full of oil it’s absolutely impossible to shove the rod back into the barrel without loosing oil. If a cylinder is retracting back in itself it has to displace oil. That could happen by either leaking out on the ground or going back through the valve. BUT a leaky piston seal will make the leaky valve problem much worse because it only has to leak the oil volume of the rod vs the whole barrel volume. It would also make the oil pressure higher since the surface area decreased.

 

[LD1]

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?

Thats exactly what they are telling you and they are correct.

I dont know why so many people have this misconception on this forum? Too many hack youtube videos or ignorant techs at a cylinder/hydraulic shop that always give the default answer "rebuild the cylinders"??? I dont know.

 

[ROUSTABOUT]

I've worked on too many excavators. I hate getting this swarm of "I'm so wrong" answers, but let's see what finally fixes it. See, if his valve leaks a little, and a leaking piston puts more pressure on it than it can handle, it will leak down just as he says. Whereas if the piston didn't leak, it may slowly leak down, but would be at an acceptable rate. I can get the cylinder packed for 40 to 60 dollars. Valve is a lot more if the valve leaks it will allow the displacement you say it requires to leak down. Being that the boom cylinder is the most used one on it, I stick with my previous answer. I'll spend less than 100 on that cylinder before I do 1000 on the valve. Maybe you would spend 1000 on the valve and then less than 100 on the cylinder? I've made my whole living with a service truck spending other people's money. Just a certain way I've learned to go about things. Road graders, dozers, skidders, Trac hoes, back hoes, I do not think it has ever been the valve. Sure, I've had some re worked, because they were so absolutely sure that the valve was the problem. It did the same thing, I re worked the cylinder and it was ready to go.

 

[LD1]

I've worked on too many excavators. I hate getting this swarm of "I'm so wrong" answers, but let's see what finally fixes it. See, if his valve leaks a little, and a leaking piston puts more pressure on it than it can handle, it will leak down just as he says. Whereas if the piston didn't leak, it may slowly leak down, but would be at an acceptable rate. I can get the cylinder packed for 40 to 60 dollars. Valve is a lot more if the valve leaks it will allow the displacement you say it requires to leak down. Being that the boom cylinder is the most used one on it, I stick with my previous answer. I'll spend less than 100 on that cylinder before I do 1000 on the valve. Maybe you would spend 1000 on the valve and then less than 100 on the cylinder? I've made my whole living with a service truck spending other people's money. Just a certain way I've learned to go about things. Road graders, dozers, skidders, Trac hoes, back hoes, I do not think it has ever been the valve. Sure, I've had some re worked, because they were so absolutely sure that the valve was the problem. It did the same thing, I re worked the cylinder and it was ready to go.

He already rebuilt the cylinder.

Yes....has been said several times (maybe not in this specific thread) that bad seals in a cylinder will accelerate the drift of a cylinder, but not the root cause.

And to answer your question about spending $100 on cylinder vs $1000 on a valve. The answer is neither. I know how to troubleshoot, and a boom cylinder on a mini-ex is a pretty easy one to test.

 

[Novanut 63]

Re pack the cylinder.

Had cylinder tested. Passed no problems.

 

[Novanut 63]

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.

Yes, it drops at idle. It's been sitting and the thumb has leaked down.

 

[ROUSTABOUT]

Had cylinder tested. Passed no problems.

Look up the service interval on your swivel joint.