Problem with TL420A Loader on a B20 Tractor

[b20bota]

I'm a new owner of a well used B20. It has a TL420A loader and a BT750C Backhoe mounted. It needs some TLC but generally seems to act as it should.

I have noticed a problem with the TL420A loader. The boom will usually drop as the valve spool moves from the UP position to the CENTER/NEUTRAL position. When the spool reaches the center position the valve does lock the loader's position. But, as I release the control valve lever from the UP position, and the spool moves back toward Neutral, there is a point were the hyd fluid in the boom cylinders flows in the wrong direction, allowing the loader boom to drop. The dropping stops when the spool reaches its center position. This happens
most, but not all of the time, and with or without a load in the bucket. I'm wondering if it could be an issue with the internal check valve (load locking valve?) in the loader control valve.

I have some basic understanding of hydraulics, but not to a point where I understand all the interactions that may be going on within the entire system of the loader control valve, the power beyond to the to the backhoe and then around and back to the 'Hyd Block Type Outlet', etc.

I guess my questions are could this issue be caused by something other than a defective check valve in the loader control valve? Or could I at least be confident that the problem is within the loader control valve? Or, might there be something going on elsewhere in the system affecting the operation of the loader valve?

The loader control valve's spools, linkages, control lever, etc all seem to be operating smoothly and as they should. Also, the directional valve in the Hydraulic Block Type Outlet body is set in correct position for operating both the loader and backhoe.

Thanks for any comments or suggestions -

 

[TheOldHokie]

I'm a new owner of a well used B20. It has a TL420A loader and a BT750C Backhoe mounted. It needs some TLC but generally seems to act as it should.

I have noticed a problem with the TL420A loader. The boom will usually drop as the valve spool moves from the UP position to the CENTER/NEUTRAL position. When the spool reaches the center position the valve does lock the loader's position. But, as I release the control valve lever from the UP position, and the spool moves back toward Neutral, there is a point were the hyd fluid in the boom cylinders flows in the wrong direction, allowing the loader boom to drop. The dropping stops when the spool reaches its center position. This happens
most, but not all of the time, and with or without a load in the bucket. I'm wondering if it could be an issue with the internal check valve (load locking valve?) in the loader control valve.

I have some basic understanding of hydraulics, but not to a point where I understand all the interactions that may be going on within the entire system of the loader control valve, the power beyond to the to the backhoe and then around and back to the 'Hyd Block Type Outlet', etc.

I guess my questions are could this issue be caused by something other than a defective check valve in the loader control valve? Or could I at least be confident that the problem is within the loader control valve? Or, might there be something going on elsewhere in the system affecting the operation of the loader valve?

The loader control valve's spools, linkages, control lever, etc all seem to be operating smoothly and as they should. Also, the directional valve in the Hydraulic Block Type Outlet body is set in correct position for operating both the loader and backhoe.

Thanks for any comments or suggestions -

That sounds like a makfunctioning load check in the valve or possibly a spool thats badly worn. I would not suspect anything other than the valve.

Dan

 

[Smokeydog]

Pushing the loader stick forward there is down, then detent to float? There is some bucket level linkage also. Simple spool valves no load checks. See a WSM in your future. Enjoyed our B20 for over 25 years before upgrading to B26. Wife thinks it’s hers which is just fine with me.

 

[TheOldHokie]

Pushing the loader stick forward there is down, then detent to float? There is some bucket level linkage also. Simple spool valves no load checks. See a WSM in your future. Enjoyed our B20 for over 25 years before upgrading to B26. Wife thinks it’s hers which is just fine with me.

What do you mean no load checks? I would guess just about every loader valve made since about 1950 has load checks. Here is the valve parts diagram. I would be inclined to think #120 is the load check but a trip to the WSM would confirm or disprove that.

Dan

 

[b20bota]

Thanks for the comments -

Smokeydog, Yes, pushing the lever forward lowers the bucket and pushing further puts it into 'float'. As Dan points out the parts diagram as well as my WSM show check valves in both control valve sections.

Dan, Thanks for confirming my initial thoughts. It seemed like the most obvious thing to me. The check valve & spring are about $20 plus a couple of O-rings to be on the safe side.

Given the price of a replacement valve it's a no-brainer to give that a shot. I expect the biggest part of the job will be wrestling the hoses off and on.

Thanks again, John