2006 jcb 550-140 hydraulic issues

I'm a framing contractor up here in northern Alberta. I have a 2006 jcb 550-140 telehandler. I've owned it since new and have never had any major issues with it. Last Friday the fork carriage started slipping. It's not engaging without having to pull up or down substantially on the control lever. It's also not self levelling as you raise or lower the boom and then will just drop on its own. I checked the haudraulic oil level and its definitely low. I'm thinking there might be air in the system. I'm planning on changing the hydraulic oil and filters. Anyone got any insight on this type of issue? Everything else on the machine seems to be working as normal and there are no major leaks on it. Like I said I've owned it since new and it only has about 3400 hrs on it. I'm also looking for reliable source to find a Manuel for it online. I've misplaced the one that came with it. Any info would be greatly
appreciated.

I doubt its air in the system but who knows. My thoughts are you have a cylinder packing issue at the master or slave.

Hi
I guess yours has a self leveling ram a the rear of the machine to keep the forls level as you lift and lower as to speak.
The tilt ram on the carriage will also have a check valve in there big valve screwed in on the pipe work directly welded in on the tilt ram,
I would guess you have a seal gone on one of the rams allowing oil to bypass thus tipping your carriage out .