Lost all hydraulics tonight on 743. Fluid pouring out the back.

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kensteer

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Apr 5, 2023
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Tonight the machine seemed like it was low on fluid. It probably was as the lift was straining. Then I heard a "phitt" sound. I assumed it was a hose. When I lifted the cab, I could not see anything. But fluid is pouring out the back real fast and I can hear the tank draining down. I am sure all the fluid will run out.
 

Micheal

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Apr 23, 2023
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Tonight the machine seemed like it was low on fluid. It probably was as the lift was straining. Then I heard a "phitt" sound. I assumed it was a hose. When I lifted the cab, I could not see anything. But fluid is pouring out the back real fast and I can hear the tank draining down. I am sure all the fluid will run out.
There was a hose that connects to bottom of hydraulic control valve that had a hole in it. The only reason I found it was because when I had my motor out I decided to pull it and rebuild it out of bobcat. I don't know how anyone could replace that hose without taking it out. It was hard enough with motor out, can't imagine how hard, or even possible with motor in.
 

brdgbldr

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Tonight the machine seemed like it was low on fluid. It probably was as the lift was straining. Then I heard a "phitt" sound. I assumed it was a hose. When I lifted the cab, I could not see anything. But fluid is pouring out the back real fast and I can hear the tank draining down. I am sure all the fluid will run out.
There are lines that run between the motor and pump. The steel lines get a bunch of dirt and gunk packed around them and then rust through.
 

Micheal

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There are lines that run between the motor and pump. The steel lines get a bunch of dirt and gunk packed around them and then rust through.
Yes this is true.
Pulling the motor wasn't as big of job as I thought. You will also have to remove the blower housing which was a pain. Those lines run behind the blower housing.
 

Markle

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Sep 8, 2020
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Been there done that. See pic for the lines visible after the engine and blower housing are removed and after hours of degunking! Your leak could be any of these lines. I replaced the bottom 2 with the T shown as they were corroded with pinholes. A local diesel shop manufactured new steel lines (bending) using my old ones as templates…much cheaper than from dealer. If there is any doubt about hydr valve spool seal integrity, this would be the time to replace them (engine out).
 

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kensteer

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I have read that in a 743 you can replace use 1040 oil for hydraulic fluid. I doubt I can find Mobile Hydraulic fluid around here.
 

brdgbldr

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I have read that in a 743 you can replace use 1040 oil for hydraulic fluid. I doubt I can find Mobile Hydraulic fluid around here.
The real old machines used oils like 10W-40W. Then in later models the manuals only recommended it if the oil was low and you didn't have access to hydraulic/hydrostatic oil To top it off. Many people on here have said they use motor oil without any
issues at all.

I have researched many oils and the main differences are in the additives used. Motor oil and hydraulic oils use the same base oil. I would not use real cheap oils as pumps and motors are way more expensive to replace then using good oil.

If Bobcat has oil in stock at shop.bobcat.com, they ship to your door for free as long as you spend more then $99 and their oil really isn't any more expensive then other brands. You do however get a little piece of mind that the oil is high quality.

The chain case is just an oil bath so you can use just about any oil in it. I use the old oil from the hydraulics in the chain case since it has been filtered the whole time.

Here is a video that covers Bobcats new oils. Pay attention to what he says about hydrostatic systems. The last time I checked Bobcat still had the old wide temp range oil also:
 
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kensteer

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Thank you so much for the info. How do I check, add to, or replace chain case oil?
 

brdgbldr

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Thank you so much for the info. How do I check, add to, or replace chain case oil?
There is a steel plug on the front.
Park on a level surface, remove the plug. Put your finger in the hole to the first knuckle and if you don't touch any oil it is low. If you open it up and it pours out you either have a motor carrier seal blown or it has filled with water over time. If the oil is milky it needs to be replaced.

You may want to remove the top inspection plate to inspect the oil and see how much water is in it. All chain cases have water in them from condensation.

The only way to get the oil out is to pump it out. Under normal circumstances this is done through the plug hole. However, if the oil hasn't been changed in a long time you will want to get all the gunk out through the inspection plate hole. You can get a cheap oil pump online or some automotive stores carry them.
 

Topper1

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Feb 19, 2023
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We've used 0w40 synthetic in all our hydroststs for years. Wide temp range -30 to +40*C. Most rexroth & danfoss pumps and motors accept it with no issues. We always add an anti wear HTO additive to keep them quiet. Chain cases will accept almost any oil. Generally we use a 10w30 or 10w40 motor oil in all of ours.
I would recommend replacing any chain case plugs with a magnetic ones. Makes for a much easier cleanup of any filings and quick visual diagnosis of abmormal wear, should a chain or sprocket decide to quit.
 
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kensteer

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My 743 leeks a lot of fluid while sitting still and not running. I suppose it could be leaking from any number of places. It is a bad leak; with an area about on the concrete 3' x 3' in less than 24 hours. What is the most common place for it to be leaking so bad while not running?
 

brdgbldr

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Your best bet to find a leak is to clean the machine under the cab using degreaser. Then you will be able to narrow it down.
 

Dave1234

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Apr 21, 2023
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My new to me 742B is apparently drinking hyd oil. Nothing on the ground, so I assume it must be going through the drive motors into the chain case, which smells like hyd oil. Is there any way to figure out which side is the problem? Any advice? I will replace all I can on both sides, but would like to stop the leak first before overfilling the chain case.
Thanks
 

brdgbldr

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Jun 1, 2012
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1,258
My new to me 742B is apparently drinking hyd oil. Nothing on the ground, so I assume it must be going through the drive motors into the chain case, which smells like hyd oil. Is there any way to figure out which side is the problem? Any advice? I will replace all I can on both sides, but would like to stop the leak first before overfilling the chain case.
Thanks
No good way that I know of to figure out which side it's on unless you pull the motors. Could be both, if the seals are the same age.
 

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