Removing air from system

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Tazza

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How long does it usually take for the hydraulic/hydrostatic system to bleed its self?
As some of you know, i have been working on re-building a 743, the tilt cylinder is not on yet, but the rest of the hydraulics are attached. I re-built the rams, so they were both empty and all the hoses and tube lines were also empty.
I have re-built cylinders on other machines and i'm sure it didn't take this long to get the air out. I lift and drop the arms a few times, drive around a bit and lift and drop the arms a bit more. The oil is still foamy and you can hear and feel the air as the pump groans and the arms jitter while lifting. Yu can see the foam through the sight glass on the reserve tank after you give it a bit of a run.
There are no suction line filters, so it can't be a restriction from a filter. There are no oil leaks so i'm pretty sure its not drawing in air from a loose fitting...... The hose clamps on the tube running from the port block to the pump are tight and not leaking so i suspect its not sucking in air here.
The only other thing i can check is the charge pressure, but with air in the oil, i'm pretty sure it won't give me the answers i'm looking for....
Any ideas? or does this seem normal? all i can do is keep running it and see if it will purge the air on its own. The system really was bone dry, hydro filter, pumps, motors, oil cooler, ALL lines and cylinders were empty.
 

skidsteer.ca

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Tazz I would think all of the air would work itself out pretty quickly. I've never emptied the cooler and all the lines, just the reservoir and cylinder, filters. Still if the oil circulates under pressure the air must come out. as far as the cylinders after 1 or 2 cycles they "felt" normal, no pump skips. almost sounds like your losing charge pressure., or have some sort of restriction in the supply side of the system. If you are going to test run the unit more I think I'd put it up on jack stands to keep the press in the hydros as low as possible while you sort it out. Then you can drive it arond without working it. See if the foam and the skipping pump goes away.The higher the pressure when you have air trouble the harder it is on motors and pumps. If the wheels can turn free the press will stay low. However I can't see the air being trapped in the system for long. The first loop the oil makes should clean out 98% of it. I'd get a 100 psi guage and tap into the charge pump circuit and watch the pressure. Not sure on a 743, my 553 has around 60 psi, and this will dip to 25 or 30 momentarily if you bottom out a cylinder( pump "over relief"), and jump to 90 or so when you lower the boom quickly. But outside of these two exceptions stays steady @ 60 psi. Maybe the replenishing valve is sticking open and your press is low??? I've never had to fight to much with the hyd system, so I'm not much help, but this is the only other cause, that comes to mind that you have'nt mentioned. Ken
 

Tazza

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Tazz I would think all of the air would work itself out pretty quickly. I've never emptied the cooler and all the lines, just the reservoir and cylinder, filters. Still if the oil circulates under pressure the air must come out. as far as the cylinders after 1 or 2 cycles they "felt" normal, no pump skips. almost sounds like your losing charge pressure., or have some sort of restriction in the supply side of the system. If you are going to test run the unit more I think I'd put it up on jack stands to keep the press in the hydros as low as possible while you sort it out. Then you can drive it arond without working it. See if the foam and the skipping pump goes away.The higher the pressure when you have air trouble the harder it is on motors and pumps. If the wheels can turn free the press will stay low. However I can't see the air being trapped in the system for long. The first loop the oil makes should clean out 98% of it. I'd get a 100 psi guage and tap into the charge pump circuit and watch the pressure. Not sure on a 743, my 553 has around 60 psi, and this will dip to 25 or 30 momentarily if you bottom out a cylinder( pump "over relief"), and jump to 90 or so when you lower the boom quickly. But outside of these two exceptions stays steady @ 60 psi. Maybe the replenishing valve is sticking open and your press is low??? I've never had to fight to much with the hyd system, so I'm not much help, but this is the only other cause, that comes to mind that you have'nt mentioned. Ken
Thanks for the reply Ken.
I called “the man” at bobcat today, he knows 743's inside and out. He also says the air should clear its self quickly. The best way to remove the air is to run it, then let it sit then run it again. So i will give it time and see how it goes. I did try it again this morning and it did seem better, so fingers crossed......
I do think i will check the charge pressure though. I have an old gauge in the shed that will read up to about 150PSI, my manual has the charge pressure in there somewhere. If memory serves me well, it was 90-120 PSI depending on the model.
He did say another good way to remove the air is to run it at 2/3 throttle. Drive at almost top speed lifting and droping the arms but do not take them to full travel as to not go over the relief vavle. Without a ROPS on mine yet, i really don't feel safe doing that. I don't want to tip it over as there is no bobtach either to help as a counter weight.
See how i go!
 

skidsteer.ca

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Thanks for the reply Ken.
I called “the man” at bobcat today, he knows 743's inside and out. He also says the air should clear its self quickly. The best way to remove the air is to run it, then let it sit then run it again. So i will give it time and see how it goes. I did try it again this morning and it did seem better, so fingers crossed......
I do think i will check the charge pressure though. I have an old gauge in the shed that will read up to about 150PSI, my manual has the charge pressure in there somewhere. If memory serves me well, it was 90-120 PSI depending on the model.
He did say another good way to remove the air is to run it at 2/3 throttle. Drive at almost top speed lifting and droping the arms but do not take them to full travel as to not go over the relief vavle. Without a ROPS on mine yet, i really don't feel safe doing that. I don't want to tip it over as there is no bobtach either to help as a counter weight.
See how i go!
That sounds reasonable, time for the air to rise up out. And about the relief thing. My charge pressure colapses to @ 25 psi when I open the relief valve by bottoming a cylinder. Stays @ 60 (if mem serves) most of the times, and rises to 90 when lowering the boom quickly from top to near bottom. You would be vary light on the front with no QA and bucket. No air pressure required in the front tires light. I think I'd put it on jack stands and jack under the rear and make my full speed runs there. Hopefuly it comes around, after all that work. Ken
 

Tazza

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That sounds reasonable, time for the air to rise up out. And about the relief thing. My charge pressure colapses to @ 25 psi when I open the relief valve by bottoming a cylinder. Stays @ 60 (if mem serves) most of the times, and rises to 90 when lowering the boom quickly from top to near bottom. You would be vary light on the front with no QA and bucket. No air pressure required in the front tires light. I think I'd put it on jack stands and jack under the rear and make my full speed runs there. Hopefuly it comes around, after all that work. Ken

I ran it a bit more when i got home, it seems all good now. There was a small amount of jittering at first, but it seems to have cleared now. All seems well!!!! i really need to take some more piccies of it, the problem is all the work i have done and all the time it looks like i have done almost nothing. Its amazing just how long some things take, i spent no less than 2 weeks worth of evenings at the lathe machining the pins and matching bushings.
Its actually pretty stable without the bobtach attached. The rear door is not on so that does help a bit. Still, its not as stable as it is with a bucket attached but still you can move it around, just not at any great speed.
 
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