Allis 7020 Hydraulics Deadheading

I am helping out a local friend diagnose and fix his 7020.  Last year, he was round baling with the tractor and it all of a sudden bogged down and the hydraulic pump sounded like it was deadheading and not going into standby pressure mode.  He drove it 5 minutes back to the shop and shut it off.  He said he took off a valve on the side of the pump to see if it was stuck and that made no difference.  I am wondering if he may have checked a valve on the gear pump instead of the piston pump?  From my knowledge of piston pumps, I'm thinking the swash plate is stuck at full or close to full angle and not returning to its 0 degree/ 0 flow position.  I see where the cam plate pivots in the pump, but the parts drawing doesn't really make clear the spool that controls the cam/swash plate pivot.  I see also there are 2 pumps depending on the serial number, so if that is going to make a difference in diagnosing it, I'll get that information from him.

Piston that controls squash plate may have come apart. Check compensated first for stuck spool then pull pump.    MACK

Sounds good, I'll pass the word on and let you know what he finds.  

Just looked at the parts manual more, and it looks like the piston pump bolts up to the outside of the transmission housing on the front right side belly of the housing.  So it sounds like he took out the compensator valve and that was free.  Looking through the diagrams, it looks like the piston pump is on the right side and the gear pump on the left (Side with the cab door)?

The large front spool isn't the problem. It's either a false signal coming from the hydraulic valve stack, a stuck or damaged low pressure compensator spool, or the broken control piston inside the pump which sees to it the swash plate (which is spring loaded) cannot be de-stroked.

Correct.

More than likely it's the other way around, something is failing in the tractor, and the pieces are messing up the pump. 

All three times, that we had rear end failures, the hydraulic pump failed first.

All pumps that I have seen stuck was from internal pump parts blocking squash plate.    MACK

You don't need to drain the transmission. Take the left hand belly filter out and inspect it and the cavity for any filings. Usually the steering is the first to be affected by rear end filings. Leave that filter out when you remove the pump,so that the anti drain back valve keeps the oil in the rear end. It helps to have the proper fittings to cap some of the lines also. Yes,there are only the two rear end (hydraulic) filters. Two half inch capscrews hold the Piston pump to the gear pump.

That could make sense with what's happening. So it sounds like it needs to have the left filter pulled, then the pump pulled? Are there any good allis shops left neat central ky?

If one of the slipper pistons is broken and jammed in there,it usually ruffs up the swash plate. Just as well look for a good used pump if that's the case.

It doesn't sound too bad. If he's not in a hurry and I can borrow the service manual from the other allis fanatic in our small town, I'd be glad to help him year down the pump and inspect it and go from there. Right now I have one of my fords waiting on a hydraulic pump and a lot of hay to get up in the next couple of weeks, so I hate to start on another project that might sit around taken apart. That's when you end up with extra bolts!

Hurst