Hydraulics Newbie - repair or replace cylinders

[seth350]

Hmmmm....you got my gears turning Paul.

If both cylinders allow the bucket to tilt down, then it would depend if the bucket eventually stops without reaching full extension of the cylinders. Or if the cylinders extend out as far as they possibly can. This is only true for double acting cylinders suspending a load. Because the load is acting on the rod side area which is smaller than the piston side area, the oil can bypass the seal to the piston side.

The first scenario would indicate a leaking piston ring, where equilibrium is eventually reached and the cylinders stop before reaching end of their stroke. While the other would indicate a leaky SCV.

 

[sporteus]

So, if I'm following this correctly, I think I'm looking at a leaky SCV. The cylinders do extend all the way eventually and as previously stated, the fluid has to come/go somewhere, so that would be back through the valve if there are no leaks, which there doesn't appear to be.

Last question, to be sure. If there were a leak somewhere after the SCV, could it suck air (allowing the cylinder to extend) and not spew fluid when retracted? I don't think so given the pressures and other dynamics, but wanted to be sure.

Probably won't worry about it now, but maybe in the spring. Is there a "rebuild kit" for Prince control valves or is it a replacement situation? I'm pretty sure this is a Prince valve, looks to be the same as the pictures on their site including the "proudly made in america" sticker in the same location! That's pretty definitive identification, right?!! LOL.

 

[seth350]

Almost any leak can suck air, and yes that is possible for it to be sucking air in SCV. Like I said, if the cylinders do finally stop before reaching end of travel then your valve and hoses are holding tight. This is because of the cylinders reaching a equilibrium state where pressure is the same on either side of the piston and they hydraulically lock. This piston side would also pull some vacuum, so as long as there is no where for the vacuum to suck in air or oil and there is no where for the rod side oil to go, then the cylinder would stay locked.

One thing I have seen with spring return valves is the spring would get weak and not return the spool to center. I have got around this in a pinch by putting a washer behind the spring that is not returning the spool to center.

 

[sporteus]

Am I also correct in thinking that even if there was a leak somewhere upstream of the the control valve (between the pump/sump and the valve), the control valve should hold the hydraulic pressure in the loader cylinders as a closed circuit and the cylinders shouldn't move?

The more I think about this stuff, the simpler it gets. I'm seeing some rebuild kits for the control valves, but their like half the price of a new valve, so thinking I may want to save myself the hassle and just replace the valve. Also thinking that having a float position valve would be useful for some leveling work, the current valve does not have float position.

My obsession continues.....stay tuned.

 

[seth350]

You are correct. Doesn't matter what is going on elsewhere in the system. If the valve is functioning correct and in the neutral position, it will hold the load. IF it is a closed center valve where the piston and rod ends of the cylinder ports are blocked in the valve.

Can you post the link to your rebuild kit and valve?

 

[sporteus]

seth350, of course I didn't bookmark what I was looking at and now I can find it. Maybe I was looking at something else because all I can find right now is the seal kits, not the rebuild kits with springs and valves.

I think I've changed my priority to rebuilding the boom cylinder on the backhoe or buying a rebuilt if I can find someone in the area. It's gotten worse and is now leaving a puddle when at rest. Sigh. (And I think I'm going to have to pull the front end this winter and replace the seals on the front wheel drive. Will be posting a separate thread on that fiasco.)