I've been just living with some droopy cylinders on my old ASV (rubber-tracked loader). The tendency for them to let gravity win has survived through a couple of my admittedly-amateur rebuilds. Is this more frequently caused by a leaky control valve, vs piston seal? (There are no external leaks.) I was thinking I could clumsily verify this by joining hoses from the extend/retract ports, raising the loader-arms and then loading it to see if one or both drift when isolated, but I'd rather cut to the chase if it's likely the joystick, and see about rebuilding THAT.
If curious about cylinder condition. Remove both lines and blow air in with a air nozzle. Once rod extends or contracts there should be no more air or fluid coming out of other port. Do both cylinders. If packing is continually damaged inspect inside of barrel.
It is easier to dead end the cylinder and remove the line on that end. Push the control lever. If oil blows out the dead-ended port the piston packing is shot.
You don't even have to dead end the cylinder. Just put weight against one side and crack the opposite side hose with the engine off and pressure drained off the hydraulic reservoir. If the cylinder drifts down and oil pours out that connection, then you have a cylinder issue. I suspect you may be correct about the leaky directional control valves. How old of a machine is this and how many hours on the clock?
One method to test a cylinder is to cap off port A in the above diagram and then put pressure into port B. If there leakage at the piston packings the rod will move out as the pressure on the A side of the piston will be greater than that on the B side due to the rod taking up the surface area of the piston. There may be some movement at first toward the A side as the air in that end is compressed.
I like that idea, but have to wonder about the relatively small air-pressure it would exert to expose a leak, vs working hydraulic pressure...
OK, this method seems like something I should already have figured out...thanks. Simple and effective with minimal oil-drainage. It's an old machine - 1997 - though I bought it around 2002. Though it had last been owned by a landscaper, there was something that later clued me in to it formerly having been a rental machine, which wouldn't have been my preference. I used it irregularly and since I had to replace the non-working tach/hour-meter (had been OOC for who-knows-how-long) I can't guess at the hours. Joystick, on rare occasions, 'locks out', as if I've switched the machine off. IIRC when that first happened and didn't just stop on its own, I verified power to solenoids and that they were both working (was it two? long time ago), so I ended up never figuring that out. Now it's not uncommon for it to lock out for a fraction of a second until I bang on the joystick a couple of times with my hand. Anything to do with seal-issue there? I've no idea what the innards of that control look like.
Interesting...took me a long time to understand your explanation - I think I did, anyway. So say a terrible rebuilder left the piston off the rod when putting it back together (to model an extreme version of my hypothetical piston-seal leak) and then I did as you suggested, closing off A and pressurizing B, then the rod would shoot out of the cylinder entirely, of course to the right. If NO leaks, though, it might not move at all, or shift slightly left until air is compressed?
Yes. If the piston seals leak the pressure oil will go past the piston and cause the rod to come out, the worse the leak past the seals the faster it will move. One important thing to keep in mind when doing this you need to cap off the "A" side of the cylinder with something that can handle the full pressure of the hydraulic system, not just a thin plate of steel.