Weak Loader; won't stay lifted.

[Zete]

If you do replace the control valve assembly that should take care of the loader drifting down so quickly. 2 spool control valves with PB and SAE threads are in the $250 to $400 range. Even a new control valve assembly will let a loader drift down over a few hours, but it will have tight enough internal clearances to slow the fluid from leaving the cylinders so quickly.

You said you want to check it yourself, and with the control valve assembly doing its job you can then add a pressure gauge on a hydraulic "T" fitting which will let you figure out if the cylinder internal seals are leaking or if the hydraulic pump has gotten weak....or both.

Hmmm....come to think of it, if your 3pt hydraulic circuit is independent of the FEL hydraulic circuit you could insert the pressure gauge on a "T" fitting back there to check the hydraulic pump right now.
Your shop manual should have a pressure vs RPM curve for that pump.

But it sounds so far like you are still going to need a new control valve to stop the FEL drifting down.

BTW, how is the power steering? Any clues there?

.

Thanks, the power steering is good. I will be looking for a new valve.

 

[Zete]

This is an excellent animated video explaining why the cylinder won't drift "downward" even if the piston seals are removed and there are no external leaks including the valve. Lots of great hydraulics videos on this channel!

Edit: The irony and, perhaps, confusion, contradictions etc is that the OP's (zete) problem and symptoms may very well be caused by bad (shot) piston seals along with a leaky valve and a pump that is no longer performing to its factory specs.

So, basically what he's saying is, for the ROD to enter the cylinder, the rod's displacement volume must compress or displace the same volume of fluid in order to enter the cylinder. It cannot compress that much volume to any extent and cannot displace it because the system is closed off by the valves. So the piston/rod cannot drift, not because of the seals, but because of the displacement volume. I can see that. Ok, I'm going to start with the control valves. Thanks!

 

[Tx Jim]

Similar to what I stated on Tuesday(reply #9).
If you have a ""glass full of water or oil"' & you insert broom stick(cylinder rod) in the full glass of liquid. Broom stick(rod) will cause liquid to exit glass spilling on outside of glass

 

[LD1]

So, basically what he's saying is, for the ROD to enter the cylinder, the rod's displacement volume must compress or displace the same volume of fluid in order to enter the cylinder. It cannot compress that much volume to any extent and cannot displace it because the system is closed off by the valves. So the piston/rod cannot drift, not because of the seals, but because of the displacement volume. I can see that. Ok, I'm going to start with the control valves. Thanks!

You seem to have a better understanding now.

Now re-read my post #22.

If your seals are shot....a leaking valve can make the drift exponentially worse

 

[Zete]

I drew up this quick diagram to help myself understand it better, so I will share....
I learned something new today. Now to figure out the rest of it.

 

[Zete]

You seem to have a better understanding now.

Now re-read my post #22.

If your seals are shot....a leaking valve can make the drift exponentially worse

Got it, thanks!

 

[rjpotts]

The boom on my backhoe began drifting down (more than normal). I switched hoses between the boom and the dipper, in both cases drifting requires extension of the cylinder. The boom drifted at the same rate as before, the dipper didn't drift at all, suggesting there are not valve issues.

There are no leaks on the boom cylinder. I'm left to assume oil is getting past the piston seal and the rod seal is letting air in. I'm planning to tear down the cylinder.

 

[npalen]

The boom on my backhoe began drifting down (more than normal). I switched hoses between the boom and the dipper, in both cases drifting requires extension of the cylinder. The boom drifted at the same rate as before, the dipper didn't drift at all, suggesting there are not valve issues.

There are no leaks on the boom cylinder. I'm left to assume oil is getting past the piston seal and the rod seal is letting air in. I'm planning to tear down the cylinder.

I'm not understanding why the rod seal would necessarily be letting air in.

 

[rjpotts]

Piston seal for the win!

 

[rjpotts]

I'm not understanding why the rod seal would necessarily be letting air in.

If piston seal failure results in the rod extending the internal volume of the cylinder will increase and needs to be satisfied. Since my valve didn't exhibit any problems when I switched ports it seems unlikely that oil is is filling that extra volume. If that's correct the only thing left is air. There are two places where air could enter, the rod seal and the o ring between the rod seal cartridge / internal bore of the cylinder. If the o ring has failed it should leak under pressure but it doesn't. The rod seal works when internal pressure expands it. If the internal pressure becomes a vacuum I'm guessing it becomes less effective and could let air in?