Is bleeding necessary after replacing hose?

[frogpond]

I need to replace a damaged hose on my Kubota BT602 backhoe, leading from the block up through the boom. It controls the dump function of the bucket. I'm going to get a new one made up. I've never done this before, and i wonder if I will need to bleed the air out afterwards, and if so, how to do it. Any advice will be appreciated.

 

[ptsg]

No need to bleed. Just cycle the functions a couple of times after you replace the hose. It might take a couple seconds for the affected circuit to react but it's just taking the air out.

Check out the hydraulic fluid level in the end.

 

[frogpond]

PTSG, thank you!

 

[davedj1]

And here I thought you were talking about blood...LOL I say yes it is , not air out of the lines though, they should be fine. What I did when I replaced my lines on my woods backhoe is I tied a masons string to the hose before pulling it out, they I had the hose to measure and the string to guide it back in, did it three times out of the 5 hoses and it worked great.

 

[k0ua]

And here I thought you were talking about blood...LOL I say yes it is , not air out of the lines though, they should be fine. What I did when I replaced my lines on my woods backhoe is I tied a masons string to the hose before pulling it out, they I had the hose to measure and the string to guide it back in, did it three times out of the 5 hoses and it worked great.

That sounds like a great idea.

 

[frogpond]

And thank you, davedj1. I followed your idea of attaching a line to the old hose to pull the new one through. Totally effective in avoiding bleeding of blood, as well as cursing. As I couldn't find string, I used some wire.

 

[CoyPatton]

No need to bleed. Just cycle the functions a couple of times after you replace the hose. It might take a couple seconds for the affected circuit to react but it's just taking the air out.

Check out the hydraulic fluid level in the end.

I agree with the no need to do any special actions to bleed the system. But terminology wise I would say that hydraulics are usually a self bleeding system. A couple of examples that you want to eliminate as much air as possible from would be the hydraulic filter (filled it as full as possible when replacing) and any thing to do with the suction/supply line going to the hydraulic pump. The filter is often in this line. Air going to the pump can (highly likely) can cause captivation. This can quickly destroy your pump.

 

[davedj1]

And thank you, davedj1. I followed your idea of attaching a line to the old hose to pull the new one through. Totally effective in avoiding bleeding of blood, as well as cursing. As I couldn't find string, I used some wire.

I'm glad it worked out for you :thumbsup:

 

[sparc]

I recently changed a line to the boom cylinder and afterwards the boom now sags under it's own weight. Never had a problem with this cylinder until I changed the hose. Now, even after a few hours of use it has not self-bled and returned to normal. I have down pressure and I can also raise the boom, it just will now no longer stay raised.

And to top it off I have the same symptom in one of the outriggers but in reverse, and I never touched any of the hoses on that cylinder. I can lower the outrigger by gravity I'm guessing but have no down-pressure to lift and level the machine. And I have very little pressure to raise the outrigger. It's like the cylinder is full of air but I never broke a line or any connections on that circuit.

I was wondering if air is in the valve body as the controls are a high point in all the circuits. But the left outrigger, dipper, and bucket all work fine with plenty of power.

Any ideas? I need to get this thing back to work before it gets cold here. I've got work to do for my brother-in-law on his property digging up 10 tree stumps for him and I don't want to be out there in the cold. I hate the cold.

 

[Tx Jim]

sparc
More than likely boom lowering after R&R of line is coincidental. My guess problem is control valve is leaking between spool & housing

 

[CoyPatton]

It should be self bleeding, but require many more cycles than you would expect to complete bleed the line. You could also have a cylinder allow fluid to seep past seals internally allowing sag or fluid seeping past seals in the control valve allowing sag. Most folks with older equipment either lower the bucket/boom to the ground/trailer or chain it up for storage or transport.

 

[sparc]

"It should be self bleeding, but require many more cycles than you would expect to complete bleed the line."

Thanks for the replies. If what you say is my problem then is there any way to speed this up?

If I leave the boom extended (which is the hose I replaced) and more or less laid out flat on the ground could that help bring the air up out of the cylinder to where it will bleed back to the reservoir. Or am I thinking 180 deg out and it's the opposite side that needs the air removed? That doesn't make sense to me as when the boom is raised that cylinder is almost vertical and the hoses connect on the lower end so any air is trapped in the cylinder.

It may seem like I'm just thinking out loud here so to speak but the sooner I get this working the better so if you have any additional ideas or suggestions I would appreciate them.

 

[CoyPatton]

"It should be self bleeding, but require many more cycles than you would expect to complete bleed the line."

Thanks for the replies. If what you say is my problem then is there any way to speed this up?

If I leave the boom extended (which is the hose I replaced) and more or less laid out flat on the ground could that help bring the air up out of the cylinder to where it will bleed back to the reservoir. Or am I thinking 180 deg out and it's the opposite side that needs the air removed? That doesn't make sense to me as when the boom is raised that cylinder is almost vertical and the hoses connect on the lower end so any air is trapped in the cylinder.

It may seem like I'm just thinking out loud here so to speak but the sooner I get this working the better so if you have any additional ideas or suggestions I would appreciate them.

Repeated cycling is the best way to ensure that air travels throughout the circuit involved.
If your control valve has a relief/bypass for complete extension/retraction you can hold the valve for a several seconds on each cycle.
Is that the only valve that seems to be affected?
It is unusual for pressure lines to suck air, and unusual for suck air on the supply side to the pump (the usual area to suck air into hydraulics) to only affect 1 valve.
Have you checked your hydraulic fluid level since starting operation of the hoe after hose replacement?

 

[sparc]

Repeated cycling is the best way to ensure that air travels throughout the circuit involved.
If your control valve has a relief/bypass for complete extension/retraction you can hold the valve for a several seconds on each cycle.
Is that the only valve that seems to be affected?
It is unusual for pressure lines to suck air, and unusual for suck air on the supply side to the pump (the usual area to suck air into hydraulics) to only affect 1 valve.
Have you checked your hydraulic fluid level since starting operation of the hoe after hose replacement?

Well, it's been a few weeks and though I haven't used the BH recently I have put about 10 hours on the BH since I last posted and still have the problem with these two cylinders.

I did check the fluid level and it was low but not so low that it might have been sucking air, only took a gallon and the reservoir holds over 10 gallons, that was the first time I added hydraulic fluid in 9 years.

I can understand a problem with the boom but the outrigger has me baffled since I didn't do anything to that circuit. Since these cylinders are power in both directions, no fluid actually (barring a piston seal leak) normally moves from one side to the other. If the valve is tight and no leaks in hoses it seems to me the only place hydraulic pressure can be going is past the piston and leaking to the other side which would let the boom dip and have reduced power. But if that were the case wouldn't it also be true for the other direction? But it would have to be a serious leak for the boom to drop like a rock, not slowly. From fully raised unless I am applying power to raise the boom it will drop to the ground in about 5 seconds. The one outrigger is the same, falls to the ground under it's own weight in about 10 seconds but it also has no power to raise the BH.

If I change the hose on the boom and it is not bled or filled before put in operation (it's only a 3/8 hose about 5 feet long so very little volume) could that slug of air be enough to get in the manifold and affect the outrigger which is the valve section located right next to the boom IIRC. I was careful to keep everything clean and capped/plugged all openings to prevent contamination/leakage while I went to get a replacement hose from a local hydraulics shop. So having taken those steps it doesn't seem likely that anything got in the system that could have damaged a pressure seal in the valve of the cylinder.

Looking back I wonder if I should have bled the line before I put it to work.

Can I troubleshoot the piston seal by disconnecting one of the lines and pressing up the other side. If the piston leaks by I should see fluid on the vented side.

If I try this do I need to cap the disconnected hose? If I leave it disconnected and drop the end in a bucket and then see fluid from the valve body when I press up the other side that would indicate a leak at the valve wouldn't it?

Sorry if these are silly questions but I'm not good at diagnosing hydraulic problems and don't know anyone who does.

Don't have money to hire someone to fix this. Have to figure it out on my own with help from the hydraulic gurus here on the forum.