I'm having an odd issue with the remote hydraulics on my 8070.  It seems that the system gets air in it after a period of time where the tractor has sat for a few days or so.  I've read thru the shop manual and can't really seem to uncover any info that pointed me in the right direction.  This one has me stuck as I've never really had any issues with the hydraulics on any of our 8000 series.  

Under 1500-1550 engine RPMs, the remotes always function as expected with the expected amount of pressure.  Above 1500-1550,  there is no pressure at the remotes.  If I have the tractor on an implement that uses a good bit of flow,  I can hear what sounds like air moving thru the system for a few seconds. This happens when I use the remotes for the first time and after that, the hydraulics work as expected at any RPM.  

If I'm using it on something that doesn't use much hydraulics,  say a fertilizer spreader with only a very small cylinder to disengage the web, the air never seems to clear and you have to idle the tractor down to get any flow.  

There are no visible oil leaks or anything like that,  so I am pretty well stumped on this one.  Have any of you seen this before?

Thanks!

Thanks Dr. Allis!

I just rescued this tractor from the weeds this spring and after a paint job immediately changed all filters and fluids.  I never used it before changing everything so I don't really know if the issue existed before I changed filters.    

Thanks very much for everyone's replies.  It's truly appreciated.

The couplers may weep a little.  Hours on the tractor is a bit up for debate.  The hour meter says ~3500 at this point, but I don't know if I 100% believe it.  I don't think it's high hours because there is no wear on the drawbar, 3pt hitch or shifter gates on the powershift.  So who knows, the hours meter could be correct.  I just have a hard time believing that I cut a 3500 hour tractor out of the weeds.   

The couplers were pretty dirty and kinda rusty from sitting for years with no use I did my best to clean them up.  I would assume they are the factory original.  Should I replace them with AGCO or something else?  They don't work as freely as I'd like anyways.  This will be a good excuse to replace them.

The rear end is definitely full into the top glass.  I double checked this morning just to make sure and it's sat still since Saturday afternoon at this point.  

I initially thought about the compensator, but the fact that it works fine after I "get the air out" or whatever is happening has me confused. 

The FWA solenoid was leaking oil a few weeks ago and my dad discovered that the valve assembly was loose in the valve body.  He just needed to tighten it and it stopped leaking.  Most of our work with this tractor has been PTO and drawbar so far so we haven't used the remotes much until now.  I used it on the grain drill last weekend and it made the "air clearing noises" and worked as expected after that.

So,  my question is,  could a leaking FWA valve have been allowing air into the system?  If so,  I am willing to believe I cleared the air on the grain drill and now that the FWA valve isn't leaking no more air got in.  But that's all speculation.

Thanks! 

I took the engine up to 1800, and the standby pressure was around the same but 0 pressure on the remotes.  I made a hose with two male pioneer fittings and put it on the middle remote.  This allowed me to clear the air out of the system and at this point and the deadhead pressure at 1800 RPM was 2800.  

This made me check more thoroughly for leaky couplers.  I did this by hooking up a hydraulic motor I had laying around from a grain auger and the bottom remote, which is the small leaver beside the seat has a pretty good drip to it when using it.  The tractor has the Allis-Chalmers style couplers and they're rusty from sitting outside with no plugs for who knows how many years.  Should I replace them with the newer Pioneer type or rebuild the AC ones?