763 Hydraulic Oil dumps to Chain Case

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bttpe

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Joined
Dec 4, 2020
Messages
42
This is a problem I've had on both a 753 and now a 763 that I have never resolved. The hydraulic oil just completely dumps into the chaincase in a matter of minutes. I've literally gone through 60 gallons of oil on these two machines and have not resolved the problem. I'm going to work through it until I've completely solved it and then post a video on Youtube and a thread on any site / forum. Here is what I understand. The carrier seal separates the drive motor from the chain case and is prone to failure. I've both carefully slipped the seal over the shaft and then drove it home as well as just used a tool to drive it home. I had our local Bobcat Service Center install them and they simply drove them in using a welded pipe with a rod. They didn't take any special care on the shaft side although they did ruin one while doing this and had to replace it. Now is just a list of things we have tried. 1. Check the hex bolt inside the bearing shaft to make sure it is tight and not allowing fluid transfer. 2. R&R both motors looking for major scoring or damage to the geroller assembly and reseal with kit. Some threads tell us that if the the gerollers aren't tight that they have failed. I doubt that is exactly correct. It would seem to me that some leakage would be normal wear (could be wrong). However, scoring from debri would be a definite bad sign. 3. Check bearing shaft on chaincase side for movement. Looking for no more than 0.001" Should be tight like a new bearing. Otherwise must replace that bearing inside the chain case (big job) 4. Removed all inline hydraulic filters. These are the small bronze looking filters. We just removed them to verify good flow through the lines. 5. We don't have a way to measure flow for "case drain". For those who don't know some hydraulic oil will bypass or leak through the geroller assembly and go into the cavity between the motor and the side of the chain case. With the machine not moving there is almost always zero leakage. Once you try to move the Bobcat forward or backward you will see oil come out of that case drain. I don't know what is acceptable vs not acceptable. I can tell you that we ran the engine/hydrostat and motors out of the Bobcat and never saw case drain. However now that it is installed there is steady stream (don't know pressure or flow yet). 6. One of the case drains is directly connected to the case drain on the hydro pump. We question if this "fights" each other and builds pressure causing a carrier seal failure. The only way I can see to determine this is if we completely drain the chain case and look down at a new seal to see if it is pushing through. We have not done that yet. All we know to do is to repeat this entire procedure. If anyone sees something we are doing wrong let us know. Remember I'm documenting this so i can help others. If you can weigh in or persuade a Bobcat Tech to weigh in please do it so we can document a solution. Special thanks to this site: https://www.youtube.com/watch?v=koXFbOd0BKc He's trying to help and I recommend his site. The more subsribers he gets the more likely he'll continue to post helpful videos. Please help if you can.
 

Lowe.Buuck

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Joined
Aug 23, 2017
Messages
61
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If you are blowing that seal, then either:
1. You have excessive leakage that the case drain lines are unable to handle.
2. There is not excessive leakage, but a case drain line can not flow the volume due to a restriction or blockage.
You could add a tee fitting at the case drain filter and install a pressure gauge.
If one of those lines deteriorated to the point where a flap of rubber is acting like plug it could cause a pressure spike. I have seen rubber brake lines with this condition cause loss of brakes. The line looks fine from the outside. You would have to remove the line and carefully inspect.
 

Davross22

Member
Joined
Sep 3, 2017
Messages
16
Hi Gents

I was keen to know if there was fix for this ?

I agree with Lowe.Buuck. The case drain is overlooked regularly. the pressure will have to be tested at the drive motor and other intervals until its determined where and if this is the case. What we tend to forget is the old hoses sit in the one location until their moved for what ever reason. they can flake off at certain intervals along the length. another thing to check is the 40 micron metal gauze filter if the machine has one. it maybe blocked thus causing pressure down there. all liquid finds the weakest link.

i hope this assisted in some way
 
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bttpe

Active member
Joined
Dec 4, 2020
Messages
42
First thanks to both of you for the reply. I hope I can communicate the problem well enought to help others that use this site. The input I get really makes a difference in resolving some of these complicated issues.
Interesting replies. We have not resolved it yet. I have the top cover off and all of the oiled dumped (20+ gallons). Up on jack stands we can force the fwd/rev levers and do not see hyd oil passing by the carrier seal. We did see a little oil leak out on the floor (the drain plug is removed). We did pull case drain hoses off both side. On the right side we did that at the connection to the upper resivoir. The filters are removed and it flowed 'alot'. On the left side the hydrostat case drain connects to the motor case drain. We pulled that filter out and have flow both ways. a) flow out of the hydrostat case drain b) flow out of the drive motor case drain. I've tried a tee fitting here connecting to the resivoir and it didn't fix it. Notable when we bench tested this setup there was zero hydrostat case drain flow and zero motor case drain flow. That would not be same today.
Up next we will install a tee fitting on both sides and record pressure of the case drains. Then we are rebuilding the drive motors using seal kits from Loader Part Source. I'll remeasure the pressure on both sides (filters out and filters in) and report what is happening. Thank you.
 
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bttpe

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Joined
Dec 4, 2020
Messages
42
Ok I may have this solved although I'm not sure what fixed it. I took the drive motors off again and the Carrier Seals installed by the Bobcat Service Center failed. This doesn't surprise me. They were very nice and helpful and I have nothing but good things to say about them (Bobcat of Columbia, MO). Their installation method surprised me. They had a simple welded tube that fit on the carrier seal and drove it into place with a hammer. They let me watch and ask questions. They did destroy one seal going in and replaced it at their cost. We put the drive motors on and ran it a couple minutes before running all the oil into the chain case again (grrrrrr). When we disassembled another seal was destroyed and the evidence was the spring around the seal off and twisted. We replaced the seal using our method which is a 2" PVC sewer pipe wrapped with tape on the end so that it expands the seal to slide onto the mating shaft. IMO Bobcat should make a tapered tool to do this. Because we had multiple failures we also resealed both drive motors with kits from Loader Parts Source. Together those repairs seem to have fixed the problem. We are limping the Bobcat around the driveway with the chaincase inspection cover off and pushing/pulling the drive levers with a log chain and our feet (I don't recommend this it is not a safe practice) However, we didn't see anything up on jackstands and wanted to move around under load. Fingers crossed we are putting it back together and will give it another try.
 

koko

Well-known member
Joined
Jan 30, 2022
Messages
76
Ok I may have this solved although I'm not sure what fixed it. I took the drive motors off again and the Carrier Seals installed by the Bobcat Service Center failed. This doesn't surprise me. They were very nice and helpful and I have nothing but good things to say about them (Bobcat of Columbia, MO). Their installation method surprised me. They had a simple welded tube that fit on the carrier seal and drove it into place with a hammer. They let me watch and ask questions. They did destroy one seal going in and replaced it at their cost. We put the drive motors on and ran it a couple minutes before running all the oil into the chain case again (grrrrrr). When we disassembled another seal was destroyed and the evidence was the spring around the seal off and twisted. We replaced the seal using our method which is a 2" PVC sewer pipe wrapped with tape on the end so that it expands the seal to slide onto the mating shaft. IMO Bobcat should make a tapered tool to do this. Because we had multiple failures we also resealed both drive motors with kits from Loader Parts Source. Together those repairs seem to have fixed the problem. We are limping the Bobcat around the driveway with the chaincase inspection cover off and pushing/pulling the drive levers with a log chain and our feet (I don't recommend this it is not a safe practice) However, we didn't see anything up on jackstands and wanted to move around under load. Fingers crossed we are putting it back together and will give it another try.
Following this thread closely, not because I'm having any of the same issues but just to be informed, education is priceless! To bttpe, would you mind posting a couple pix of your seal driver? Please & thx..(y)
 

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