This just happened today. They were working fine last week. They put a little pressure on the ground but won't raise it off the ground. I checked fluid level and I had to add about a gallon but I had the loader extended and the boom and dipper. If anyone could point me in the right direction I'd appreciate it.
I should add that they retract normally and quickly.
I'm assuming that all the other functions work normally...the boom, dipper and bucket...also the loader? Is this issue happening on both sides or just one? Assuming that the hydraulic pump didn't just go overnight, which CAN happen, this sounds like it could be a check valve problem. I'm not familiar with a 480C at all. I have a 580B. I'm currently diagnosing a similar situation on my backhoe. None of my backhoe functions work. Machine has been sitting for about 3 years and was working fine when parked. I rebuilt the swing cylinders 3 years ago and just put them back on a month ago, only to find that the backhoe doesn't work. Sorry if I'm being longwinded, just trying to explain the process I went through and what I've found. I've been troubleshooting for a month and just today remove my main valve block. When I took out the check valve from one of the sections, it was missing an O-ring, both on the upper and lower. I know your stabilizer control valve isn't the same as the backhoe valve body but it DOES check valves in it. I have yet to do anything with mine as I'm rebuilding the whole valve bank and I just got it out and disassembled today. I would imaging the lack of O-rings on a check valve might produce symptoms similar to what you describe. Just a thought...
The hoe is fed through the loader control valve, on the outlet side of the loader control valve toward the fuel tank where the hose goes to the hoe circuit there is an oring that seals on the fitting and the valve interior to direct oil flow through the valve and out to the hoe valve, those have been known to fail , usually the hoe will have some movement but no power. That is the first thing to check. When this fails the hoe will work normally but the hoe will be sluggish.
I was told this same thing by Tinkerer. This was indeed the first thing I did. My seals on the power beyond fitting were good, but since I had it apart I replaced them. No difference.
All the other functions were working until today. I extended the boom and went to retract it and it extended further. It just keeps extending no matter which way I pull the lever. I'm kind of in a pickle now.
So, I noticed the grass was wet under the tractor. The fitting on the loader supply line was leaking. Tightening it didn't help. I pulled the hose off and it had been cross threaded on. I had to pull the fitting off the control valve. That meant pulling the fender because the fitting is a 90 and there is no clearance to turn it with the fender on. I thought engineering was better back then. Anyway, got the new fitting on and the boom works now. Also noticed the stabilizer that won't lift is now leaking so that needs resealed.
Good to hear you got it working.