BT 900 backhoe problem

TheOldHokie

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L3901/LA525, B7200DT/B1630, G2160/RCK60, G2460/RCK60
Apr 6, 2021
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Myersville, MD
windyridgefarm.us
does not seem to swing full amount in either directions, drifts back to center somewhat. more so going left than going right. i just rebuilt them this summer along with my loader cylinders. did not fix the issue. This issue was happening before i rebuilt the cylinders. at this point im trying to find the time to swap lines around i suppose, try to see if its in the control valves. if so, i found a good rated hydraulic shop 2hrs away that can rebuild it. kits and parts are discontinued.
Even with leaking seals a cylinder can not drift in the closed (retract) direction without some sort of external leak. Since those cylinders are opposed and have been rebuilt once already kind of rules that out and leads me to suspect the valve.

There is only so much a rebuild shop can do for the valve. They can replace rubber seals but that is not what holds the cylinders. The cylunders are held by the clearance between spool and bore in housing. If thats worn the spools will leak and the cylinder will drift. At that point its time for a new valve. Another possibility for valve leakage is the port reliefs. If they are dirty or worn they will not hold pressure and the cylinders can drift. Good news is they can be replaced. Bad news is they are usually pricey.

To check for valve leakage just disconnect both hoses from the ports on the swing cylinder valve and plug/cap the hoses. If that stops the cylinder drift the valve is your problem.

Dan
 
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Nicksacco

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Kubota L35 TLB, 2014 RTV-1140CPX
Sep 15, 2021
678
382
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Bahama, NC
@JonF I've just gone thru a long process of replacing pins and rebuilding cylinders on my L35/BT900.
The plumbing of the swing cyl is simple to see, but confusing to explain.
The cylinders are plumbed such that when one cyl is extending, the other is retracting - but you probably already know that. I'm including info for others here that may have issues with BT900 or just curious.

Just because they have been rebuilt, doesn't mean that the cylinders or other parts aren't damaged. Some thoughts for you:
1) The nuts holding the piston to the rod could be loose.
2) The plumbing could be incorrect somewhere (or damaged)
4) Piston seals could be improperly installed causing internal leaking
5) the swing boom itself could be damaged. (Or the pins/bushings)
5a) the boom should move rather easily side to side when the cylinders are not pinned to the boom.
6) Your actual controls may be worn out/mal-adjusted such that they do not depress the valves correctly
7) Make sure that the control for the Backhoe is pushed all the way in. The one in picture 5
8) Swing CYL bushings rusted not allowing free movement.
9) Low/dirty hydro fluid / dirty filter
10) Pump has low volume (is loader working ok?)
11) Connections to/from backhoe are worn/leaking/clogged/misaligned.

It appears you have new pins on the swing cylinders where they meet the boom. Do the cylinders move side to side easily when disconnected?

The swing cyl have large shafts that are mounted into bushings on the bottom of the swing frame and held in place in the top by the large, thick bar (again there are bushings in that top bar that mate with the shafts on the top of the cyl). These could be horribly rusted not allowing free movement.

The seals for the cyl may be NLA, but are plain old seals any hydro shop can match. The hoses between the cylinders are NLA, but I think I found the last in existence via Messicks - but I wanted OEM because I really didn't know how long to make them. I can give you those measurements if you need them.
What's critical are the 90 deg fittings because there is not much room in the swing body;

On MY tractor however the bottom of the CYL double nipples are already 90 deg so the hose fittings are straight.

Plumbing is as follows:
1) Easy to get confused on LEFT and RIGHT.
2) TOP Hose from valve goes to LEFT CYL top double nipple (EXTEND LEFT - RETRACT RIGHT)
3) BOTTOM Hose from Valve goes to RIGHT CYL top double nipple TOP (EXTEND RIGHT - RETRACT LEFT)
4) BOTTOM LEFT CYL Double nipple goes to RIGHT CYL TOP single nipple
5) BOTTOM RIGHT CYL Double nipple goes to LEFT CYL TOP single nipple


swing plumbing.JPG



NOTE THE DIFFERENCES BETWEEN CYLINDERS - RIGHT is upside down.
On BOTH CYL Double nipple EXTENDS the Rods.


SWING CYL.JPG


THIS IS THE TOP BAR WITH BUSHINGS

TOP BUSHINGS.JPG



BOTTOM OF MY TRACTOR's double nipples are already 90 deg.

Bottom of double nipples.JPG



Directional Lever components.JPG
 
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GreensvilleJay

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BX23-S,57 A-C D-14,58 A-C D-14, 57 A-C D-14,tiller,cults,Millcreek 25G spreader,
Apr 2, 2019
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Greensville,Ontario,Canada
Wow Nick, I'm impressed with the detail and pictures you've posted ! I can see how easy it'd be to turn the 'simple' job into a friggin NIGHTMARE.Attaching the hoses must have been 'fun'.

curious #1. is there easy,known access to that 'top bar with bushings' so the trunnion pivots can be greased? looks like Zerks might be there?

curious #2. there's a long 'stud' on each cylinder,facing down. does a cover plate attach onto them ?
 

Nicksacco

Well-known member

Equipment
Kubota L35 TLB, 2014 RTV-1140CPX
Sep 15, 2021
678
382
63
Bahama, NC
Wow Nick, I'm impressed with the detail and pictures you've posted ! I can see how easy it'd be to turn the 'simple' job into a friggin NIGHTMARE.Attaching the hoses must have been 'fun'.

curious #1. is there easy,known access to that 'top bar with bushings' so the trunnion pivots can be greased? looks like Zerks might be there?

curious #2. there's a long 'stud' on each cylinder,facing down. does a cover plate attach onto them ?

Hi Jay, Thank you!

In the first picture, you can see the yellow caps where the zerks are. Definitely located in the top for greasing the cylinder's trunnions. You don't need a ton of grease since it will just squirt out and lay on top of the bar (not doing any good). I don't yet have the 4 bolts installed in this pic.

You can clean the threads of the zerks using a 1/8 x 27 NPT tap.

Removing the top bar is easiest if you raise a stabilizer about parallel to the ground. Then you can lift it off the cylinders and slide it out over the stabilizer cylinder.

No, getting in between the cylinders to tighten the hoses isn't impossible, just a pain. Get a good (no, GREAT) headlamp!

To my knowledge, there are no covers or anything mounting to the cylinder bottoms. They would likely interfere with the cylinders anyway. I used a silicone powercoating nipple to cover those threads on the bottoms of the cylinders. I think they are just made that way perhaps because the cylinders were used on other machines.

I did post a cover setup for the lower grease trunnions here:

I also use Mystik (Green) JT-6 grease. It's clear and sticky. Some of the other greases that are dark just looks dirty all the time and very difficult to clean. At least I know when the Mystik needs replenished.


Completed swing system.JPG