# BT7500 Backhoe boom



## JHenders (6 mo ago)

The BT7500 Backhoe boom on my B20 swings right but not left. Everything else functions fine. Has me baffled !!!! Any ideas?


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## pogobill (Jan 31, 2012)

Welcome to the forum. Any odd sounds or perhaps a disconnected linkage causing you trouble?


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## HarveyW (Sep 15, 2014)

Both swing cylinders are powered during a swing - Does it swing right as normal, or is it slower than normal?? If it is slower, I would suspect a bad swing cylinder. If all seems normal on the right swing, my guess would be the control valve.


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## JHenders (6 mo ago)

HarveyW said:


> Both swing cylinders are powered during a swing - Does it swing right as normal, or is it slower than normal?? If it is slower, I would suspect a bad swing cylinder. If all seems normal on the right swing, my guess would be the control valve.


Thanks for your response. I would suspect a bad cylinder if it swung slower but it doesn't swing left at all. It swings right normally so fluid is coming thru the valve (at least for one direction.) The valve looks to be ok and hasn't been tampered with.


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## FredM (Nov 18, 2015)

I would be inclined to think the cylinder piston seals have failed, not much to go wrong with the spool valve.

Does your machine have two 4 way control levers?, if 4 way, lift the rubber dust boot and check that a couple of set screws are tight, if loose, the spool wont travel fully.


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## JHenders (6 mo ago)

If one of the piston seals went bad I would think that the boom would move slightly since both cylinders are powered. My machine has 4 way valves but I don't see a rubber boot; is it on top or bottom? I disconnected the hydraulic line between the spool valve and cylinder and fluid appears to be flowing!


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## FredM (Nov 18, 2015)

JHenders said:


> If one of the piston seals went bad I would think that the boom would move slightly since both cylinders are powered. My machine has 4 way valves but I don't see a rubber boot; is it on top or bottom? I disconnected the hydraulic line between the spool valve and cylinder and fluid appears to be flowing!


If there was a rubber dust seal boot, this would be on top with the lever running through the boot, can you attach a photo of your control levers, I have tried searching for your backhoe but one can't be sure if what I see is correct, while you are at it, could you attach a photo of the boom ram, do you know if the ram is single acting or double acting?.

There is a possibility that the piston has parted company with the piston rod.


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## JHenders (6 mo ago)

Hi FredM, Thanks for sticking with me on this problem. Per you request, I have attached a pic of the spool valve and piston. Hopefully they will be helpful. Both look fine to me i.e














. not broken, bent, leaking etc.The boom cylinders are both double acting. I'm thinking that even if one cylinder is shot the other cylinder should move the boom?


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## Fedup (Feb 25, 2014)

Maybe if we had the model number of the backhoe attachment? It might help to study the parts layout of your machine, the hydraulic parts, lines, valves, etc. Need a model number for that.


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## FredM (Nov 18, 2015)

JHenders said:


> not broken, bent, leaking etc.The boom cylinders are both double acting. I'm thinking that even if one cylinder is shot the other cylinder should move the boom?


That is if both the boom cylinders are double acting, if they are single acting, then the boom will only move one way, like Fedup said, time for diagrams/parts breakdown.


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## JHenders (6 mo ago)

The backhoe attachment is a Kubota Model BT750. Where does one find a piping schematic?


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## Fedup (Feb 25, 2014)

JHenders said:


> The backhoe attachment is a Kubota Model BT750. Where does one find a piping schematic?


Don't know about a piping schematic, maybe Kubota has a manual covering the repair and maintenance on these machines. I doubt I've ever seen one. Usually when something breaks it's kind of self explanatory. The parts website shows a diagram of the swing cylinders, along with the hoses, fittings, routing, etc. That might tell you something. You can access that online at the Kubota parts website if you wish. It's one of those that won't let me link to it, you need to do it one step at a time.

It seems the two cylinders are parallel plumbed, with one rod extending, the other retracting whenever you swing the boom. It could be that something inside one of the cylinders has failed as Fred suggested. Whatever that is may now be acting as a sort of check valve, allowing pressure to build in one direction so the boom swings that way. Then in the other direction the fluid is leaking by, finding it's way back to return without doing any work. One possibility. 

Another of course is the valve. Is it actually sending fluid under pressure in both directions on demand? We're not really sure of that yet are we? When the boom reaches full right and stops, do you hear the hydraulics load up and trigger a relief valve? You should.


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## FredM (Nov 18, 2015)

The only other thing I can suggest is to buy a 3000PSI pressure gauge with fittings to test both outlets (actually both are inlet and outlet, depends on which way the lever is moved) on the right hand swing spool valve, the gauge will indicate if the pressure is correct, if you go this way, don't hold the lever forever on, pull and watch the gauge needle to where this reads and release, doing this will show if it is a pressure problem or mechanical.

I have tried Kubota parts and they don't seem to show a BT750 hoe, so that is as far as I got there, Messicks have a breakdown of a BT750B hoe, I believe this is much earlier than yours.

Probably if we could stand beside the machine, we would most likely pick up the problem, not so and we are just keyboard warriors trying our best.


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## JHenders (6 mo ago)

I switched the "swing boom left" hose with the "boom raise" hose and sure enough the boom swings left now but does not raise. This tells me that the problem is in the spool valve and not the cylinders. Are rebuild kits for spool valves available? Is it a complicated job?


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## FredM (Nov 18, 2015)

JHenders said:


> Hi FredM, Thanks for sticking with me on this problem. Per you request, I have attached a pic of the spool valve and piston. Hopefully they will be helpful. Both look fine to me i.e
> View attachment 80937
> View attachment 80938
> . not broken, bent, leaking etc.The boom cylinders are both double acting. I'm thinking that even if one cylinder is shot the other cylinder should move the boom?


JHenders -- you wont be able to replace the spool by itself, you will have to buy the spool and body as a set as each set is machined and polished to fit as an assembly, and to replace, you will have to remove the complete control bank set and dismantle, you will need to buy the full O ring and seal set for when the banks are reassembled, you should replace the O rings to each spool valve top and bottom while this is apart, check with the supplier if the O rings are included with the kit for the spool valves with the control valve bank seal kit so you don't double order.

Before you dismantle the bank, is it possible to remove the lever from the swing spool so you can feel for resistance either way, if you pull the spool up by hand you should feel resistance and if you push the spool down you should feel resistance, engine off of course, there is a spring setup at the bottom of each spool bank to centralise the spool and you need to feel if this is working, if the spool returns to center only one way, then it is possible that the spool has broken, don't dismantle anything bank wise until you check and double check that swing spool.

If you were to remove the two slot head screws from the swing spool valve O ring retaining plate and were the spool to be broken, (degrease the top of the banks and blow dry before removing the plate, you don't want grit or the like going into the spool body) then you would be able to pull the spool out, if you feel the return spring resistance, then the spool is intact and we will have to look further.

Should you go down the path of dismantling the spool bank, can I suggest that you number each one in sequence and to keep each spool with their respective bank body, you can't mix the spools with a different body.

This is when a parts breakdown is required so it can be seen what the system entails.

Have you thought of writing to Kubota with the tractor and backhoe details with what you have said in your last post here and asking them if they could advise you on what the problem may be?.


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## JHenders (6 mo ago)

Hi FredM,
Thanks for your advice. As you suggested, I disconnected the lever and pushed and pulled the spool. In both cases there was resistance and the spool returned to center. I also removed the "o" ring retaining plate and tried to pull the spool out of the body. There was resistance from the spring so I guess that means the spool is not broken. I was hoping that I would not have to rebuild the valve but that may be the only option. It still seems strange to me that even if the valve is leaking that there wouldn't be at least some pressure to the boom cylinders.
Regards,
Jim


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## FredM (Nov 18, 2015)

J Henders, can you show which valve bank controls the boom swing, should the outer banks be the one, then remove the pressure relief valve and check the workings of the valve, don't undo the top locknut and set screw yet, inspect first, undo at the body to remove from the valve bank, We are running out of options, when you consider that the valve bank is pressure fed and this pressurised oil is supplied to all of the bank through galleys cast into the bodies, so unless some great piece of foreign matter got into the oil somehow to cause a blockage at that particular bank, to which I doubt very much, then the only other option is the pressure relief valve has failed, I don't know how this would be when the valve is there to prevent overload to the system when the load being used, the ram being fully extended or retracted or a resistance in the work area.

So that is all I can offer, I cannot think of anything else, have you completely swapped a hose over to check if the hose hasn't failed internally and creating a blockage, then again, if there was a blockage in the hose, as soon as you loaded the hose up, the engine and relief valve would let you know.


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## JHenders (6 mo ago)

Hi FredM,
Thanks for your input. As you suggested, I removed the relief valve from the valve bank. It looked ok but the pin inside didn't move in or out as it probably should. I then swapped the relief valve with the one next too it (for the "boom up & down") and sure enough the 'boom left & right" functions now work but the "boom up & down" don't. So it looks like the relief valve is the problem. Do these things stick? Can I soak it in penetrating oil to free it up? Do they wear out? Should I just buy a new one? Many thanks for your logical thoughts and sound advice


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## FredM (Nov 18, 2015)

JHenders said:


> Hi FredM,
> Thanks for your input. As you suggested, I removed the relief valve from the valve bank. It looked ok but the pin inside didn't move in or out as it probably should. I then swapped the relief valve with the one next too it (for the "boom up & down") and sure enough the 'boom left & right" functions now work but the "boom up & down" don't. So it looks like the relief valve is the problem. Do these things stick? Can I soak it in penetrating oil to free it up? Do they wear out? Should I just buy a new one? Many thanks for your logical thoughts and sound advice


Wow!!, finally found the problem, I am sorry for not mentioning the relief valve earlier in my writings, never mind we have got you on the road again.

You can dismantle the relief valve to inspect and clean, when you undo the pressure adjusting lock nut, just crack this enough to unlock the spring tensioning socket head rod at the top, you will need to keep the lock nut and rod at the same position when re-assembling, unscrew the rod to relieve spring tension and then dismantle the body taking note or lay each piece on a tray to keep in order, wash in unleaded fuel and air dry if you have a compressor, don't wash with water.

Would you mind attaching a photo of the dismantled relief valve when you get this far?.

As for parts, I am not sure if you would have to buy the full assembly or if you can buy the valve body and seat separately, you will have to send off an email to the Kubota shop and ask, if parts are needed.


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## JHenders (6 mo ago)

Hi FredM,
I cleaned up the relief valve as best I could (carb cleaner & compressed air) without taking it apart and it now seems to be working fine. As long as it's working, I'm happy so I think I'll leave it. If the valve ever jams in the "open" position again I'll take it apart and go from there. Thanks again for all your help - you're awesom!


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## FredM (Nov 18, 2015)

That is great news, you are on your way again, strange the valve was stuck in the open position, you will be right in keeping those pressure relief valves in mind for future reference, top of the world to you!!.


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## brownsbotab20 (3 mo ago)

new to the forum, i have an issue that involves a bt750 backhoe attachment. can i add an additional hydraulic outlet to the spool so i can operate a thumb for the backhoe. also can i shim the main hydraulic pressure valve to increase the hydraulic pressure for my 25 year old tractor?


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