Bobcat 7753 dumping hydraulic oil into chaincase ONLY under heavy load – carrier seals done, case drain filters replaced, stumped

[smolar]

Hey all — first time skid steer owner here and I’m getting pretty discouraged, hoping someone can point me in the right direction.

I recently bought an old Bobcat 7753 super cheap. Overall the machine was actually working well (drive, lift, tilt). BOSS system is toast and bypassed (it was shutting the engine down due to sensor/code issues), and AUX isn’t wired up / not tested.

The big problem I can’t solve is the machine dumping the hydraulic reservoir into the chaincase.

Work performed so far​

• Pulled both drive motors
• Replaced carrier seals (3 times total — I re-did the first one I installed because I wasn’t fully confident in it)
• Resealed both drive motors (new seals, springs, etc.)
• Replaced both case drain filters (less than 1 hour of operation ago)

Also: I am not able to turn the drive motors by hand (I read on another post that if you can turn them by hand, the motor may be dead due to excessive geroller play, so I’m hoping this is normal for a healthy motor?).

Current symptoms / testing results​

Here’s what has me confused:

1. Installed carrier seal, parked machine level
2. Drained chaincase to the fill hole
3. Filled hydraulic reservoir
4. Drove lightly for 15–20 minutes just driving around
   ✅ Checked chaincase: NO extra fluid (thought it was fixed)

Then:

1. Ran the machine “hard” for ~25 minutes (pushing dirt and logs)
2. Heard the low hydraulic oil noise from the drive motors
3. Hydraulic reservoir is empty
4. Chaincase is full

So it seems like it only happens under higher load / higher pressure, not during light driving. That’s why I’m thinking this is a case drain pressure issue, not just the carrier seal failing again, and the drive motors appear to work well - it drives straight (one side is not pulling, etc,.) and they have plenty of torque, etc,.

What I have NOT done​

I did not remove / reseal / inspect the drive motor shuttle valves (I’ve seen them called shuttle valves / bypass spool / etc.). I’ve read that if those stick or fail they can cause excessive case drain pressure, which could force oil past the carrier seal into the chaincase.

Next troubleshooting ideas​

I’m considering:

• Install a pressure gauge on each motor’s case drain line using a T-fitting to see if case drain pressure is increasing under load
• Take both motors to a hydraulic shop for testing (but $$$, trying to avoid if possible / don't want to go down the drive motor rabbit trail if it's actually a case drain/hydraulic hose problem)
• Inspect/replace all case drain hoses, because I’ve also read hoses can fail internally and restrict flow → causing case drain pressure to rise

Questions​

1. Does this symptom pattern (light drive OK, heavy use dumps into chaincase) point more toward:
   • bad/failed shuttle valve?
   • internal motor bypass (excess case flow under load)?
   • case drain restriction (hoses/fittings/filter head)?
2. Is there a known “normal” case drain pressure value for these motors?
3. Any recommended tests I can do before paying for a full hydro shop diagnosis?

I knew this would be a project and I’ve learned a ton… but it’d sure be nice to use it for more than 30 minutes without dumping $100 of hydraulic oil into the chaincase

Any advice appreciated — thank you!

 

[Tyler d4c]

Too much case pressure the rotating assemblys are worn harder running aka high pressure causes higher case pressure/flow then the case drain system can flow and it is blowing past the seals could also be pump case pressure over runming the case drain (not sure if they are teed in to the same line leading to the tank) you have atleast one motor or pump that is over flowing the case drain you need to issolate and test case flow from each componet to see who the bad one is

 

[Tones]

Tyler is correct.

 

[smolar]

thanks so much for the replies - I will do some testing on the motors and see what I find // take them to be tested by a hydraulic shop. Appreciate it a ton.

 

[smolar]

Okay I am back. I ordered and installed two new final drive motors (OFF BRAND because of parts availability but they seem to work just fine) -- Machine appears to have plenty of power; I used it for 30 minutes and noticed oil leaking. The O rings (at least 1, maybe more) on the high pressure lines (#12 // 3/4 inch) near the hydrostatic pump BLEW and oil was leaking/spraying out.
Tonight I replaced all 4 O rings and refilled oil - after 10 minutes, they were blown again and oil spraying on to the bottom of the seat and everywhere.

#1 Maybe I did not use the correct O rings for this -- I think they were the correct size, but I used generic O rings from harbor freight - maybe I used ones that are not 'good enough'?
#2 I torqued the fittings 'hard' but maybe not enough? I have read that for o-ring face seal fittings, you don't want to OVER torque them, because that can crush the o-ring and ruin the seal.

Only other comment is: The new drive motors do have 2 case drain ports, but I only connected 1 of the ports (per motor) and left the other port plugged on the motor. I read that is generally fine to do since the ports should be tied together internally, and I don't think this is possibly related to my high-pressure circuit blowing, since a case-drain failure would leak into the chain case//past the carrier seal rather than blow an o ring on the high pressure.


Any suggestions or thoughts would be greatly appreciated.

 

[willie59]

All the o-ring type hydraulic adapter fittings on that machine are what's known as SAE o-ring boss (ORB) fittings. Those fittings require a specific o-ring, 900 series. All SAE o-ring part numbers start with 568, then you have the series number, 568-100 for 100 series o-rings, 568-200 for 200 series, you get the idea. So for SAE o-ring boss, if you have a -8 fitting, the o-ring will be 568-908, a -10 will be 568-910, any other o-ring used will fail. I've said this to machine owners before, if you're going to own a machine like yours with o-ring boss fittings, go online and find an o-ring boss o-ring kit, you're going to need it.

 

[smolar]

D’oh! Thank you so much! This is exactly what I needed to know. I just ordered the o ring boss kit (buna hardness 90) and will get them all replaced.

 

[willie59]

Correct, ORB o-rings are typically 90 durometer. And another thing about ORB fittings is installation. Straight fittings aren't a problem, it's simply screw them in and lock them down. It's the 45 degree and 90 degree fittings with a washer and jam nut that get installed incorrectly, and believe it or not, I've taken loose factory (manufacturer) fittings that were installed incorrectly causing o-ring failure. It's actually simple, but important. On 45 and 90 fittings with washer and jam nut, fitting in hand, back off jam nut. You'll note the washer fitted to a recess on the fitting. Push the washer by hand, I typically use a 12 point socket that fits over the threads of the fitting to push the washer, along the recess until it bottoms out at the threads the jam nut is fitted to. Now run the jam nut down until it just touches the washer. Install the fitting turning it until the fitting and o-ring seat by hand. If at that point it's indexed properly to connect mating hose or pipe, fit it up and lock down the jam nut. But if it stops short of indexing properly, rotate the fitting back out, index it to connect hose or pipe, then lock down the jam nut

 

[Welder Dave]

Yeah, on high pressure hydrostats use only high quality proper o-rings. If they blow you probably need to check the pressure from the pumps isn't higher than the factory spec. It's annoying when the drive motors leak into the chaincase. Expensive too as often times the motors have internal damage and need replacing.

 

[smolar]

Thank you all so much for your help - I am happy to report that after installing the O rings (and then doing it a second time to torque those fitting down HARD - 60ft lbs is what I found online ) and fingers crossed I have not had any more spraying or chain case leaks!

Now I need to deep clean this disgusting machine - I was thinking purple power in a had sprayer, soak it for 10 min then rinse off with garden hose. Repeat as needed.


Also this machine had the BOSS computer system that is “bad” - display never worked according to the previous owner and half of the wires are bypassed or unplugged. I have “hot wired” the ignition directly to the motor- can you all suggest what gauges etc I should look at adding?

-engine temp
-engine oil pressure
-fuel guage
-hydraulic temp? Pressure?
-battery voltage?

I may have to fabricate or 3d print a new guage “housing” for the cab.

There are also zero safety systems set up right now - I like to call the “hot seat” - it sounds like a lap bar would be sufficient but I appreciate your comments and suggestions.

And there is the matter of the auxiliary hydraulics… not sure how involved it is to rewire for that - as I understand it, it’s basically several switches (aux on, aux mode, then joystick controls for flow) which control solenoids, but the joystick buttons are trash and I’ll need to figure something out if I want to get the aux working again…

I saw a post about replacing the boss wiring and it sounds like a “kit” used to exist but on an old machine like this I may be better off just piece-mealing it by hand.

 

[Welder Dave]

I think the gauges you mentioned would be sufficient. I'm glad I didn't buy a Bobcat with the early BOSS system. Have heard nothing but horror stories with it.

 

[smolar]

Okay, I'm back. The problem now is Drive motor stuttering / insufficient power/torque. Here's the post I tried to put on a facebook group, but I'm cross-posting here -- little more details about the machine, etc,.

I have an older Bobcat 7753 (Essentially a vertical lift 753 as I understand it) - history unknown, but it was in rough shape when I got it. The BOSS computer is basically bypassed and not functional, the AUX fittings at the end of the lift arms don't have quick attach fittings (so somebody took them off), sensors removed, etc,.

1st BIG problem was hydraulic oil dumping to chaincase. Ended up replacing the drive motors; seemed to fix it, but I made a rookie mistake and failed to torque the drive motors down correctly; they started leaking at the motor mounts. Tightened down and that fixed that.
But, I noticed passive hydraulic oil leaking -- removed the drive motors and sure enough, the carrier seals were BLOWN - I suspect from the 'loose' drive motors allowed high pressure oil to blast the carrier seals... But it's also possible that a case-drain circuit over-pressure caused damange, too.

Replaced carrier seals, reinstalled motors and torqued to spec (100 ft lb); added 5 gallons of hyd oil, ran OK for 5 minutes, let it sit all night - no leak.

Went to run it and push dirt and now I have a new problem: Drive motor stuttering. Like if there were air in the drive circuit -- I put it on blocks and ran the motors forward and reverse and they felt perfect -- no stuttering at all. Ran it on blocks for 10+ minutes - that should have evacuated air if it was in the system.

Sooo I am now at a bit of a loss -- could it be a case drain pressure problem? I changed the case drain filters recently, but NOT since I replaced the carrier seals. The case drain routing is a bit odd -- left motor goes to the hydrostatic pump, while the right motor goes to the hydraulic tank. And both case drain circuits have multiple tee fittings, and have connections from other components (lift arms control box and the primary valve manifold)

I think there is still a passive leak because the dip-stick was low -- maybe my carrier seals blew again?

In this crude diagram, the ORANGE line is coming off the lift-arm/aux control box -- the diameter of the hose suggests it is a case drain, but if it were supplying pressure to the case drain circuit, that could cause all kinds of problems. I don't think I have ANY check-valves on the case drain circuits.

Lastly, here is a photo of one of the carrier seals - you can see how damaged it was.

 

[willie59]

Not saying it's your problem or your fix, but something to consider. Many hydraulic systems have pump(s) that draw oil from a reservoir, pump sends the oil to control valves, control valves send oil to functions. Oil that returns from functions goes back to tank to start over again, a true "return to tank" system. Not so with Bobcat machines like the 7753. They use a hydraulic loop system. Once oil is charged in the system, pump, valves, functions, return goes right back to pump to start the loop over again, it does not go back to tank. The tank (reservoir) is there just to replenish or exhaust the system as cylinders move in and out. This especially applies to the drive pumps/motors. The charge pump draws oil from reservoir, part of the oil it pumps is used to charge the drive pumps with oil with a charge inlet relief set at 155-165 psi relief valve keeping a charge of oil under pressure to feed the drive pumps. Once you move the drive sticks, the oil is sent out of the drive pump in one direction to the motors, the return oil from the motors goes right back to the drive pump charge cavity, just continuous loop of oil in the drive pumps/motors. Any air in the drive pump/motor loop takes time to work out through the case drains, which can take some time. But do make sure your case drains are clean and flowing oil. Also, make sure the suction hose and piping from reservoir to the pump inlet is solid and tight, a leak in the suction line can cause air ingress in the system.

 

[Tones]

AKA Closed Loop Curcuit verses Open Loop Curcuit

 

[smolar]

UPDATE: The stuttering was a loose drive belt and not hydraulic.

I fixed the Chaincase leak by correctly plumbing the new drive motors (2 high pressure ports, 2 low pressure case drains; one is some kind of hot-shuttle port with an OEM 65 psi pressure relief valve that I took off the old motor).

After getting that done, and the carrier seals installed CORRECTLY (it is so easy to mess up the seal during installation), and I have been able to run the machine for about 4 hours of heavy use without any hyd problems.

Thanks for all your help!

I may take the old drive motors to a local shop to see if they can rebuild them correctly -- I think the sequence of events was: The old motors were bypassing internally, either due to failed/failing seals, or due to mechanical wear. When I removed the motors and replaced the carrier seals, I made the problem worse. When I tried to 'reseal' the motors by using a LoaderPartsSource.com seal kit, I did not do it correctly, and made the problem EVEN WORSE. After that, I spent a bunch of time trying to identify/isolate the problem, but in my ignorance, incompetence, and laziness, did not properly troubleshoot.

When I installed the new drive motor, I did not plumb it identically to the old motor (WHY??? what was I thinking...), and then when I reinstalled them, I had a messed up carrier seal, so the leak continued (but passively instead of actively).