1. Stick issues - Old CAT 225, with engine about 50%, boom up which operated with normal speed, then stick out, giving the stick 100% of the lever, it is still very slow going all the way out to full extension. Coming back, with the extra gravity boost, the stick comes down a little too fast, then, as the stick becomes vertical and not changing anything with the lever control, the stick will pause at the vertical, actually go backwards a touch, then kind of bang just a bit as it starts to curl inward to me. At that time, it runs slow again. So, runs fast with gravity helping, runs slow when having to lift itself. Strange pause at the vertical. i ran several times expecting to get some heat (bypassing) at the cylinder but did not feel any heat at all but machine was cold and cooler today. 2. Hydro oil - I pretty much knew what I was going to find. Oil is a gray color, far from clear. Almost seemed silver which really worried me but I ran a magnet in it and nothing was in it. Since I was planning to probably rebuild all 4 cylinders on the boom arm, I planned to discard all the oil from the cylinders, then drain the tank and refill. I know I will not get it ALL out so I planned to swap filters a few times BUT as I understand, these filters do not dry the fluid so I might want to look at further improving the fluid with a "drier" if they are made for these systems? I was told the new fluid may help many of the machine leaks by swelling the seals back up. Not sure about that but I know it needs new fluid regardless.
Sound like the cylinder needs resealed. The piston nut/bolt may even be loose. Check for drift.
http://www.youtube.com/user/letsdig18#p/search/0/yiA_0fbDhPc I stumbled on this vid of a 225D that seems to respond similar to mine other than the stick in mine does not go up near as fast. Watch the very tail end of the vid. The guy pulls the stick back to him and it nearly falls down and then bounces at the bottom. I am real curious to learn what could even cause problems like this. I would think air would cause a bounce like that but how would fluid do it? Oh, a second watch and you can see when he extends the stick, it goes up quickly, then, at the end of the extension, the stick cylinder slows WAY down to get that extra little bit. That is sort of what mine is doing.
Seems to me there has to be some air in the system. I had a similar problem with my old hitachi because of a leak in a slow return valve. I'm not exactly sure how it pulled air in but stick would drop rapidly then stop until fluid caught up.
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I think you possibly have 2 separate issues, the condition of the oil itself and also possible cylinder issues, maybe other components as well. If you are thinking on changing the oil why worry about "drying" it. I would cut the filters and have a look at what type of material you can see (either with the naked eye or a magnifying glass) in them. Think about having a sample of oil analyzed for things like silicon, iron, copper, aluminium, etc. The oil analysis will be looking for particles less than 5 microns in size, much smaller than you can see. That will give you an idea of how much contamination you have in the oil. Once you've sorted all the cylinder issues and the machine is functioning properly that would be the time to think about draining the oil before putting it into operation. Drain all the cylinders, the implement lines, the tank, pump suction, basically everything you can get old oil out of. When you refill obviously the system will need bleeding again. Keep a close eye on the tank level to make sure you're not starving the pumps while you're bleeding. If you can get a high-efficiency equivalent filter elements for every filter in the system it would help to improve the conditon of the new oil and pull out those particles that you couldn't remove by draining. Do oil sampling going forward. For $20 or thereabouts you can get really good indications of whether or not your system condition is improving.