My mystery is solved!
I could swing the boom to the left but not to the right. If the tractor was on a slop leaning right, the boom would slowly move to the right indicating the pressure relief valve was open. Many people have suggested shaking the backhoe with it tilted the other way using the outriggers fixed their problem which would apparently unstick the pressure relief valve. I tried that for 10 minutes and high RPM's, low RPM's, using the outriggers to shake the tractor left and right, shaking the boom while trying to swing it and nothing I did fixed the problem.
In my research, I found the pressure relief valve is a pilot style where this is a small hole that left the system pressure bypass the pressure relief valve and put pressure on the backside of the valve forcing it shut. There is a small lightweight spring that adds a few pounds of pressure to the system pressure helping keep the main pressure relieve valve closed. When the pressure becomes out of balance between the flow and the relief side, ie more pressure on the flow side than on the relief side it overcomes the power of the spring and opens the main relief valve. That way a small spring con control a large amount of pressure and flow. One video I found likened it to a 12v relay opening and closing a 480v circuit.
On my M59 BT1200 backhoe, the control valve spool was moving fine. I removed the plate and top shroud around the backhoe hydraulic control valve. In tracing the hydraulic hoses, I found the topside of the valve was flow to move the boom left (from the seat in the tractor) and the bottom side of the valve would have flow to move the boom right. So I knew I had to remove the top pressure relief valve. I made sure there was no pressure in the system by putting the tractor on the ground and locking the boom in the up position. I took an adjustable wrench and removed the top pressure relief valve. They use an o-ring to seal it so it was not hard to remove.
I compared the pressure relief valve (7K521-63712 per Kubota Tractor BT1200/BT1200V B21001 Control Valve [Component Parts] ## [Bt1200] | Kubota Tractor) and compared it to the new/spare boom pressure relief valve (7K521-63732) I had. The pilot has a pin at the bottom with a lightweight spring that keeps it closed and on the valve I removed, the pin was stuck open. The pin on the spare valve moved freely and it only took a small amount of pressure/pushing to open it.
On another thread, a person said Kubota told them the pressure relief valve was not serviceable. If so, shame on Kubota. I took off the assembly that holds the pin in place and sure enough inside was the main valve (looked like a cup) which was held in place by a larger spring, the pin which went through the center of the cup which was also held in place with a smaller spring. In my case, for some reason, the small spring had jumped the shoulder on the pin that would hold the pin closed and therefore was no longer pushing on the pin. Since the pin was stuck open, the main valve (cup w/ larger spring) would open and all of the system pressure would flow back to tank.
I was able to remove the small spring and reseat it on the pin shoulder and reassemble the pressure relief valve. Now the pin moves freely and I could easily push it the circuit open with my finger just like the new spare unit I had. I put it back on the tractor and sure enough, the backhoe is as good as new. The story here is that if the pressure relief valve is stuck open or the spring or the small pin is broke, your system pressure when trying to swing the boom will just go right to the hydraulic tank. The good news is that it is very easy to remove, disassemble and inspect. In my case, if I had know, I could have fixed with just 2 wrenches in 10 minutes. Now I know and will not be afraid to remove the pressure relief valves and take them apart if needed.
Message to Kubota: The swing valve pressure relief valve is $650 to replace. WTF. There is likely little that can wear or break other than the o-rings, the springs, the pilot pin or the main pressure relief cup. I cannot see any of those items costing more than $10 yet there is no rebuild kit or replacement parts available. What a major miss to help owners maintain their equipment cost effectively. I suppose an owner could try to find similar springs if one broke but why have to. Shame on Kubota if they cannot make these parts available to their equipment owners!
Below are my photos and I have videos as well if someone would like me email them.
Hope this helps others out there and I really appreciate the folks on this forum and the information I was able to gather to get me moving in the right direction! Be sure to use the swing safety pin and the boom safety latch when working around or transporting the tractor. It is very clear to me know that a small issue with the pressure relief valve means hundreds of pounds of backhoe will be in motion with no control pressure check to keep it in place so be safe.
As good as new!
I could swing the boom to the left but not to the right. If the tractor was on a slop leaning right, the boom would slowly move to the right indicating the pressure relief valve was open. Many people have suggested shaking the backhoe with it tilted the other way using the outriggers fixed their problem which would apparently unstick the pressure relief valve. I tried that for 10 minutes and high RPM's, low RPM's, using the outriggers to shake the tractor left and right, shaking the boom while trying to swing it and nothing I did fixed the problem.
In my research, I found the pressure relief valve is a pilot style where this is a small hole that left the system pressure bypass the pressure relief valve and put pressure on the backside of the valve forcing it shut. There is a small lightweight spring that adds a few pounds of pressure to the system pressure helping keep the main pressure relieve valve closed. When the pressure becomes out of balance between the flow and the relief side, ie more pressure on the flow side than on the relief side it overcomes the power of the spring and opens the main relief valve. That way a small spring con control a large amount of pressure and flow. One video I found likened it to a 12v relay opening and closing a 480v circuit.
On my M59 BT1200 backhoe, the control valve spool was moving fine. I removed the plate and top shroud around the backhoe hydraulic control valve. In tracing the hydraulic hoses, I found the topside of the valve was flow to move the boom left (from the seat in the tractor) and the bottom side of the valve would have flow to move the boom right. So I knew I had to remove the top pressure relief valve. I made sure there was no pressure in the system by putting the tractor on the ground and locking the boom in the up position. I took an adjustable wrench and removed the top pressure relief valve. They use an o-ring to seal it so it was not hard to remove.
I compared the pressure relief valve (7K521-63712 per Kubota Tractor BT1200/BT1200V B21001 Control Valve [Component Parts] ## [Bt1200] | Kubota Tractor) and compared it to the new/spare boom pressure relief valve (7K521-63732) I had. The pilot has a pin at the bottom with a lightweight spring that keeps it closed and on the valve I removed, the pin was stuck open. The pin on the spare valve moved freely and it only took a small amount of pressure/pushing to open it.
On another thread, a person said Kubota told them the pressure relief valve was not serviceable. If so, shame on Kubota. I took off the assembly that holds the pin in place and sure enough inside was the main valve (looked like a cup) which was held in place by a larger spring, the pin which went through the center of the cup which was also held in place with a smaller spring. In my case, for some reason, the small spring had jumped the shoulder on the pin that would hold the pin closed and therefore was no longer pushing on the pin. Since the pin was stuck open, the main valve (cup w/ larger spring) would open and all of the system pressure would flow back to tank.
I was able to remove the small spring and reseat it on the pin shoulder and reassemble the pressure relief valve. Now the pin moves freely and I could easily push it the circuit open with my finger just like the new spare unit I had. I put it back on the tractor and sure enough, the backhoe is as good as new. The story here is that if the pressure relief valve is stuck open or the spring or the small pin is broke, your system pressure when trying to swing the boom will just go right to the hydraulic tank. The good news is that it is very easy to remove, disassemble and inspect. In my case, if I had know, I could have fixed with just 2 wrenches in 10 minutes. Now I know and will not be afraid to remove the pressure relief valves and take them apart if needed.
Message to Kubota: The swing valve pressure relief valve is $650 to replace. WTF. There is likely little that can wear or break other than the o-rings, the springs, the pilot pin or the main pressure relief cup. I cannot see any of those items costing more than $10 yet there is no rebuild kit or replacement parts available. What a major miss to help owners maintain their equipment cost effectively. I suppose an owner could try to find similar springs if one broke but why have to. Shame on Kubota if they cannot make these parts available to their equipment owners!
Below are my photos and I have videos as well if someone would like me email them.
Hope this helps others out there and I really appreciate the folks on this forum and the information I was able to gather to get me moving in the right direction! Be sure to use the swing safety pin and the boom safety latch when working around or transporting the tractor. It is very clear to me know that a small issue with the pressure relief valve means hundreds of pounds of backhoe will be in motion with no control pressure check to keep it in place so be safe.
As good as new!
