Backhoe Hydraulic Issue

[2710Fan]

Thought I would throw this out to the "experts."

I have a low hour (lightly used..not commercial) Kubota L48...I can easily lift the rear of the tractor with a little pressure from the main boom but when I release the valve the cylinder relaxes and the tractor settles to the ground within a few seconds. (Engine running). All of the other hydraulics in the backhoe work as advertised and hold pressure even with engine off.

Is this a typically a cylinder, valve or pump issue? I offer the pump as a theory because the L48 has seperate pumps for some of the hydraulic functions in the backhoe...not clear to me which are dedicated to what. In any case I am asking advice on most likely target to pursue first. Finally any advise to troubleshoot will be appreciated.

Looking forward to your thoughts.

 

[mmurphy]

Sounds like pump is fine. Valve would be suspect. Are you sure it is centering right when going back to centered position? Another possibility is a check valve in the valve if it has one on it.

 

[kennyd]

My money is on the boom cylinder, I bet the seals are leaking/bypassing internally-not a terribly uncommon problem. The valve could be the problem, but usually it's the cylinder.

 

[dfkrug]

I offer the pump as a theory because the L48 has seperate pumps for some of the hydraulic functions in the backhoe...not clear to me which are dedicated to what.

I don't have an L48, but I sure love that machine. Your hoe is driven
by 2 hyd pumps, one for the swing, and one for the boom/dipper/bkt. At
least that is what the Kubota manuf rep tells me. If you can lift your
tractor with the boom, that pump is fine.

If the boom drifts with that kind of load on it, then it is more of an
inconvenience, rather than a problem that needs to be fixed. For it to
drift in a few seconds, you must have significant bypass in the valve,
whether or not the cylinder piston seals are leaking. If the leaks are not
external, then you don't really have to fix anything. It could be something
as simple as dirt in the load check for that valve, or a scored spool.

 

[Monster5601]

My new BH76 backhoe did something simular, it had a fast leak down. The dealer discovered a small (Kubota orange) paint chip in, I believe he said, the relieve vale.

 

[AKKAMAAN]

....I can easily lift the rear of the tractor with a little pressure from the main boom but when I release the valve the cylinder relaxes and the tractor settles to the ground within a few seconds......Is this a typically a cylinder, valve or pump issue?

Since the lifting force is generated by pressure from the capped side of the boom cylinder, the leakage must be in the control valve, most likely the port relief valve....Monster5601, had help of a GREAT tech that understood where to check for this problem...
If piston seal is ok, port relief on capped side is the one that leaks...if the piston seal is bad, the port relief on the rod side can cause this leak too....

Remember that if piston seal is bad, the rod will be the net piston, and the pressure can be more than 4 times higher than normal, and port relief open at its normal pressure setting


Kubota L48 Youtube video

 

[mmurphy]

Since the lifting force is generated by pressure from the capped side of the boom cylinder, the leakage must be in the control valve, most likely the port relief valve....Monster5601, had help of a GREAT tech that understood where to check for this problem...
If piston seal is ok, port relief on capped side is the one that leaks...if the piston seal is bad, the port relief on the rod side can cause this leak too....

Remember that if piston seal is bad, the rod will be the net piston, and the pressure can be more than 4 times higher than normal, and port relief open at its normal pressure setting


Kubota L48 Youtube video

Much overlooked!

 

[SpringBay]

I have an L48 and recently had a problem with the boom not having a lot of hutzpa in either direction. Researching it. On the control valve there are 2 relief valves (top is up), (bottom is down). If either are leaking or have a broken component inside ($171 each) It may be leaking bypass fluid to the return line. A second possibility is the spool valve itself. The last may be the cylinder. If the packings are damaged (since you say low use) you can run a test. Extend the arm all the way out and about 6 inches off ground bucket teeth. Acceptable drop I am told is 1" in a minute. Still another possible is air in the hydraulic lines. Purge by raising the front loader 3 times full and holding the up position so engine works, do same in down position. Do same with backhoe.
The three pumps... #1 and #3 are together in front of engine #2 is back further. #1 is main pump for backhoe. #2 is power steering. and #3 is swing for backhoe. So #1 is the pump driving the boom. The in and out fluid flow for the boom is controlled by the spool valve that moves and allows fluid to enter either end of the hyd. cyl. The ports are in the same housing but are not directly connected by the spool as either in/out or out/in depending on position of spool. If the spool is in neutral position excessive force on the boom arm (say a tree falls across it) driving the pressure to excess, the relief valve poppet opens allowing fluid to bypass the spool and return to the transmission (avoiding blowing the hyd. cyl). These two relief valves have an adjustment screw on top. Loosen the locking nut, allen wrench turn clockwise. Every 30 degrees is increase of safety pressure by 100 pounds. You can try it say 30 or 60 degrees if nothing changes back it off to original setting immediately and tighten the locking nut. Remember this is a safety device. I really don't know how to check the cylinder by itself. I thought of just disconnect the return line and see if fluid is bypassing the cyl packings and running out. If the upper arm stays put where you leave it unlike the boom arm falling down..they are driven by the same pump, different spool valves, different relief valves, different cylinders so that's why I feel the problem is in the backhoe boom control etc.
Hope this helps.