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SnaykeByte
Join Date: Aug 2009
Posts: 9 Greenwood, Indiana
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2009-08-25          165259

1st post, glad to be here. We have a 520 tractor that blew a return line on the power steering unit last year. We replaced the line and ever since then the steering gets all stupid and wobbly in anything above 7th gear on the road. We have a service manual, but it says nothing about bleeding the system. On looking at it closer, we noticed the lines going to the assist cylinder both go in the bottom so would that not allow an air pocket to enter when most of the oil left? There is no bleeder port on it anywhere that we have found, any help would be appreciated.......-Curtis.

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earthwrks
Join Date: Dec 2003
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2009-08-25          165265

If the system is anything like I'm used to, it sounds like a bad steering valve to me. When they start to go bad, they require a lot of input to keep them steady. Mainly what happens is the reaction or torque rod inside goes bad.
The service manual should have detailed diagnosis and testing procedures. If not you'll need to take it to a dealer or the very least a competent hydraulics shop that specializes in hydraulic steering valves like hi-los have.
10 years ago my mid-sized backhoe cost $700 to rebuild it. ....

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SnaykeByte
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2009-08-25          165266

Thank you for the reply and I would agree with you, but until we blew the return line it drove perfectly fine in ninth gear??? ....

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SnaykeByte
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2009-08-26          165290

Nobody knows how to get it out? ....

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Murf
Join Date: Dec 1999
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2009-08-26          165293

Standard hydraulic bleeding of lines, same as fuel injectors or brakes.

Find the highest point in the circuit where there's a fitting and with pressure applied to one side bleed the return line, repeat until nothing but fluid comes out, no further air bubbles.

As a note though, even if there is air in the system, it will self-bleed during repeated cycling. With the engine at a fast idle steer it from fully turned one way, to fully turned the other way repeatedly. Air is less viscous than hydraulic fluid and will be displaced quite quickly by pressurized fluid. In some cases you need to take the weight off the front wheels in order to reduce the hydraulic back-pressure, but that's not the norm.


CAUTION: Live hydraulic lines can release enough pressure to seriously injure a person. Do the bleeding with extreme caution and be sure to wrap a thick rag or something around the fitting and wrench to contain the spray safely.

Best of luck. ....

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SnaykeByte
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2009-08-27          165302

That is EXACTLY my problem, there are NO fittings to bleed it at. The lines both go into the bottom of the cylinder. I am aware of how to bleed hydraulics, but there is no where to bleed it. ....

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Murf
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2009-08-27          165310

Quote:
Originally Posted by SnaykeByte | view 165302
That is EXACTLY my problem, there are NO fittings to bleed it at. The lines both go into the bottom of the cylinder.


Maybe I'm missing something, how can it have lines but no fittings?

You say you blew a line and replaced it, so there must be fittings at both ends of the lines.

Best of luck. ....

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SnaykeByte
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2009-08-27          165312

If I can figure out how to post a pic on this board I will. The two lines on the little hydraulic cylinder most certainly do have lines on it, but the lines are on the BOTTOM of the cylinder, ie, there are NO fittings that I can use to bleed the air out of this cylinder. ....

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Murf
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2009-08-27          165313

Quote:
Originally Posted by SnaykeByte | view 165302
I am aware of how to bleed hydraulics, but there is no where to bleed it.


Ah, got it now, sorry, I was mislead by your earlier comment.

Although it might work better higher up, like the other end of those lines, the lowest place in the system will still work just fine.

Air will always move before, and faster, than more viscous liquids like hydraulic fluid.

All you need to do is get enough of an opening in the system that the air gets forced out.

Best of luck. ....

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SnaykeByte
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2009-08-27          165314

Air won't do that in this shop. That's why bleeders in calipers and wheel cylinders are always in the top. You can have air in the high parts of a hydraulic brake system and unless you bleed it out it will be in there for the life of the vehicle, Air will NOT move down and flow out the bottom of this horizontally mounted cylinder unless we turn the tractor upside down. ....

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Murf
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2009-08-27          165315

Quote:
Originally Posted by SnaykeByte | view 165314
... Air will NOT move down and flow out the bottom of this horizontally mounted cylinder unless we turn the tractor upside down.


We'll have to agree to disagree then.

According to a hydraulics manual I have "Most hydraulic systems are of the closed, airless type; they are designed to self-scavenge free air back to the system reservoir. Because free air resulting from maintenance actions or other causes may enter the system at a point remote from the system reservoir, the system should be extensively cycled with full power to transfer air to the reservoir, where it can be bled off."

And if you you're ok with the results from WikiAnswer dot com, " Question: Bleeding air out of tractor hydraulic cylinder procedure? Answer: Air will be trapped in a cylinder when it is new , or when lines are replace. The only method to clear it out is by a bleeder on the cyl (if it has one, unlikely), or to cycle the cylinder to full stroke and then allowing the system to rest. This is because air will become part of the whole system for a period and you want it to go to cavities in the tank where it will bubble up and go to the environment. So start the system, cycle the cylinders and shut the system down to allow the foamy oil to clear, then do it again until satisfied."

The emphasis in both quotes is mine.

Best of luck. ....

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kthompson
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2009-08-27          165316

Not a hyd expert but have replaced a few hoses on my equipment and never had to bleed a system, just run it. But forget that and I will ask on brake systems on autos and pickups. They are bleed at the lowest point of the system and no they do not roll the vechile upside down to do so. Well here we don't. :) I have bleed more than one set of those and can not think of one reason it would work with brake fluid and not with hyd fluid, is there? ....

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earthwrks
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2009-08-27          165318

Kenny I agree with you, I haven't either which is why I say there is something wrong with the either the steering valve or its pump.

(Pssst, pssst, Kenny--over here--the past tense of "bleed" is "bled"--make a note of it 'k?) ....

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SnaykeByte
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2009-08-27          165321

Quote:
Originally Posted by kthompson | view 165316
Not a hyd expert but have replaced a few hoses on my equipment and never had to bleed a system, just run it. But forget that and I will ask on brake systems on autos and pickups. They are bleed at the lowest point of the system and no they do not roll the vechile upside down to do so. Well here we don't. :) I have bleed more than one set of those and can not think of one reason it would work with brake fluid and not with hyd fluid, is there?



In Indiana, the bleeders on wheel cylinders and calipers are on the top of said device. I wish I could post a pic so you fellas could understand what I'm saying, it isn't your fault that I can't explain it right and I appreciate the responses. Consider this: you have a double acting hydraulic cylinder that is mounted horizontally. Now, the two pressure (and return, depending on the piston movement) lines are entering the side of the cylinder directly on the bottom. The cyl bore is about 3" and the stroke is also about 3". The two lines that hook to it are about 3/8" id and about 14" long. My point is, if there was air trapped in this cyl, since it is so small by fluid volume when it travels full stroke one way or the other the air would certainly be pushed out the respective line and port. The problem with that is that the air is never really pushed all the way down and through the lines and the volume of air that is then contained in the repective steel line will just be drawn right back in the cyl where it was in the first place as soon as the piston moves the other way. In my way of thinking, with a cyl so small the air contained in the lines alone would be more than enough to cause the symptom. When tractor is running sitting still and you turn the wheels, there is what I call a "delay" in the actual movement of the wheels like the system is being "cushioned" by the presence of air, just like a spongy brake pedal if the system contained air. ....

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kthompson
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2009-08-27          165325

You may be 100 and 10% correct on all of this...but I find it hard to believe you can not drop or wrap a heavy rag around one fitting that you have loosen and work the cyl so that is the return side and push the cyl rod fully that direction and tighten that fitting and do the same on the other side. It might take a time or two but if air is really still captured in the cyl as you believe it is. Again not saying you are not correct on what you are saying. But if that is totally true then how did the manf get the air out? I worked for a major heavy equipment manf for a few years and those machines had total hyd steer and we never bleed any cylinder and we build a few thousand. I inspected many of those machines and test loaded a few.

Ok...less assume you are 100% correct, can you remove the cyl with hoses still connected? If so then roll the cyl over and then the fittings are no longer the "lowest" point but the top position.

As to the brakes bleeding...even in my older balding head am right sure the Fords we had the valve was in the center of the wheel cylinder. Those were not disk brakes. Those were the days US built heavy autos with steel dashes! No need for seat belts, the dash would stop you. Leaded gas and only two grades. LOL Sad to say I remember when gas cost less per gallon than the tax is on it today.
....

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earthwrks
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2009-08-27          165326

When I wote service manuals for convertible casr with high pressure hydraulics (3,500 to 4,200 PSI) we told the techs to remove the cylinders from their mounting and tip them so that the air would go to the highest point then cycle them several times. We were using opaque hoses so you could see when the air left the system--you won't be so lucky.

When our plant was under the gun to get product out and even the manfacturer of systems, we sometimes used a clean bucket filled with new oil and submerged the disconnected cylinder in it and manually push/pulled it to suck in/out oil. That way we were assured there were no air bubbles in it.

But I still say it's the control valve--especially now that you say there is a delay. The delay means that the torque rod inside the valve that rotates a cylinder with holes and passages in it--is slow to respond meaning it's bad and/or seals have gone bad. That perceptible lag is more pronounced the faster you go because the same but small steering input has more of dramatic response at speed--not unlike twitching the wheel of a car in a parking lot at 2 MPH has little effect on direction--but do that at 60 MPH and you'll change two lanes pronto!

Trust me, my backhoe used to do the same thing--wanader. My tire tracks looked like I was drunk. ....

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SnaykeByte
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2009-08-27          165329

I am wondering how the manufacturer does it too. The lines are steel and not flexible, so no I can't dismount the cylinder and invert it to purge the air. I may be completely off base in my diag anyway, but it's my best guess. The only basis of my theory is the fact that it drove completely fine as fast as you want to go BEFORE the top drain pipe that was made of plastic sprung a leak because it had rubbed against one of the cyl mounting bolts for years. It lost about 2-3 qts of oil, so we replaced the line with a piece of rubber hose, filled it back up and we were on the move again. The only other thing that is different is the piece of rubber hose I mentioned. Is it possible that this rubber hose is pulsating or something causing the problem? The original one was hard plastic. This line returns the spent oil coming out of the control spool back into the tank. ....

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hughesla
Join Date: Oct 2010
Posts: 3 Alabama
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2010-10-16          174608

I have a Belarus 570 and also blew a hydraulic line on the power steering. I am having problems finding the connector to the bottom cylinder. Could someone tell me what the part name for this and if anyone has a parts book the part number? Thanks alot. ....

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