743b chainbox filling with hydraulic fluid !

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HalEcopter

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Joined
Jul 29, 2022
Messages
5
Hello I have a 93 743B and the chainbox keeps filling with hydraulic fluid. I have had the drive motors rebuilt and new seal kits installed. I have installed the upgrade style motor carrier seals. This is my 3rd attempt to remedy this problem . When I pull the drive motors the motor carrier seals are damaged and pushed inward towards the chainbox allowing hydraulic fluid to enter. This must take a high psi of fluid to accomplish. Any ideas where I'm going wrong. Thanks
 

brdgbldr

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Jun 1, 2012
Messages
1,165
My guess is that possibly your bypass valve is not working. Thus, too much pressure is building up and blowing your seals.
 

Mister Gadget

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Joined
Jul 19, 2022
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4
Make sure the small case drain lines on the motors are not blocked for some reason, that would do it. If they are clear then the motor seal job failed due to, too much oil bypass in the worn out motors..
 
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HalEcopter

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Joined
Jul 29, 2022
Messages
5
Make sure the small case drain lines on the motors are not blocked for some reason, that would do it. If they are clear then the motor seal job failed due to, too much oil bypass in the worn out motors..
I will check the drain lines . The motors are worn 3k+ hours on them. I had a very reputable hydraulic shop rebuild them and I was told they had wear . Trying all options before the expensive replacement but it seems it's to that point. Thank you for the advice
 

Mister Gadget

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Jul 19, 2022
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I will check the drain lines . The motors are worn 3k+ hours on them. I had a very reputable hydraulic shop rebuild them and I was told they had wear . Trying all options before the expensive replacement but it seems it's to that point. Thank you for the advice
3000 hours is not that much. Under normal conditions the motors would be fine. I have a 1986 743, I purchased as a basket case in 1991. I know mine has over 7000 hours on it, not exactly sure because the hour meter wasn't working for about 3 years. You might want to have an oil analysis done on your hydraulic oil if not recently changed. Contaminated oil will definitely wear out your pumps and motors.
 
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HalEcopter

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Jul 29, 2022
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Hello can anyone tell me what the small hoses coming off the hydraulic drive motors attach to. It looks like a pressure relief valve but I can't find it on the schematics
 

Mister Gadget

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Jul 19, 2022
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It should be a clear path through the oil cooler, filter and back to the hydraulic pump.
IMG_7384.jpeg
 
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HalEcopter

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Jul 29, 2022
Messages
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Thank you for your help ! I found a very informative article online about the return lines and how a clogged inline filter will build up pressure after researching the issue from your suggestion. I found an aftermarket or not original to this machine filter was added and it was clogged with debris. I have ordered the replacement filter and will reassemble with new motor carrier seals when they arrive and let you know if it remedies the problem. Thanks for your help !
Bobcat Final Drive Failures and Case Drain Filters
 

River Fluid Power

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Joined
Aug 18, 2022
Messages
107
Make sure your motors have this bushing in the front, and that the o-ring seal is in place near it. There has to be a seal and bushing for the back of the output shaft that this motor plugs into.
753B motor.JPG
 

Marcus67

Member
Joined
Aug 20, 2022
Messages
5
Make sure the small case drain lines on the motors are not blocked for some reason, that would do it. If they are clear then the motor seal job failed due to, too much oil bypass in the worn out motors..
Does a 743 have a case drain filter or just the hydraulic filter in the rear of the machine. Thanks in advance.
 

brdgbldr

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Jun 1, 2012
Messages
1,165
Does a 743 have a case drain filter or just the hydraulic filter in the rear of the machine. Thanks in advance.
There is an in-line hydrostatic filter.

Go to : bobcatpartsonline.com
and enter your machines serial number and the search "filter".
It will bring up all the parts diagrams with the word filter in them and also highlight those parts in the diagrams.
 

Marcus67

Member
Joined
Aug 20, 2022
Messages
5
There is an in-line hydrostatic filter.

Go to : bobcatpartsonline.com
and enter your machines serial number and the search "filter".
It will bring up all the parts diagrams with the word filter in them and also highlight those parts in the diagrams.
Thank you very much. I'll do that.
 

brdgbldr

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Joined
Jun 1, 2012
Messages
1,165
I've had this old 743 for four years and I've been flying by the seat my pants when I needed parts. This will help a ton. Thanks again.
Yeah, I use this site a bunch. Also, I look up parts on:

to get an idea of prices and availability of parts.
 
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