Stuck spool valve on I-600 backhoe mounted on HD4

I'm a 39-year old university educated man without a lot of mechanical experience, but I'm good at figuring things out when I have to. That's just a disclaimer to indicate my rudimentary level of knowledge and that I may be unfamiliar with what are thought to be common terms regarding the kind of work I'm trying to do. Bear with me!

I recently picked up this old girl, a 1966 AC HD4 with loader and an AC Industrial-600 backhoe. She runs great and I've been working on doing a full service to her (replacing all fluids and filters) since I'm not sure when these things were last done. I should note that I have the operators manual and service manual for the HD4, as well as the service manual for the backhoe (but no operators manual).

I've lightly used it for a couple hours at a time to start digging up some stumps with the backhoe. I probably got about 6 hours in with no issues over 3-4 sessions.

However, when I went to use her for some stump digging again the other day, I ran into a big issue. I was pulling in the dipperstick on the hoe a little bit and when I released it all of a sudden it wouldn't return to neutral like it normally does. The dipper just kept coming in and wouldn't stop, and once it fully retracted you could hear the strain on the hydraulic system as it keeps trying to pull the dipper in. No amount of pushing/pulling on the lever arm resulted in it being able to return to neutral (or push the dipper out). It would appear there is a problem with the spool valve.

I should note that the lever (and the spool valve) can still be pulled out a little bit (as if you're trying to retract the dipper faster) but when you let go it just stops and won't return to neutral, not even when you try pushing hard on the lever. At the very least this indicates that the spool isn't pinched or otherwise seized as it can still move 'freely', it just stops at some kind of obstruction instead of travelling all the way back to neutral (and beyond) like it should.



I got underneath and it was pretty grungy under there, but in this photo you can see the guilt valve sticking out further than the rest.

I got out the pressure washer and gave everything a good spray down to clean up the area. After many hours of hardship,l I finally managed to get the pin out that holds the rod that holds the control levers in place, and disconnected all the control lever linkage from the ends of the spools.



I suppose my question now is what to do at this point. Research so far has led me to believe that I need to pull out the spool and clean it/check for obstructions etc. Probably needs new o-rings etc. But before I go any further I'd like some confirmation from knowledgeable folks that I'm on the right track.

Also, I'm feeling a bit discouraged at the moment because the next foreseeable step is to remove the cap for the spool and/or the relief valve, which is on the other side of the control valve block from the side the control levers were connected to:



In order to get at these things, I have to be on my back under the machine, reaching way up, because there's no other way to access it. What's more, is that there's not a whole lot of clearance between the ends of the caps and the back wall of the HD4 itself, so even if I were to get the cap off I don't think I'd be able to pull the spool out all the way before it's hitting that obstruction.

I'm really hoping there's a way to fix this without having to detach the backhoe from the machine, because I don't know how I'd go about doing that with the limited resources I have. I doubt I'd ever be able to get the hoe back onto the machine if I took it off.

Sorry for the novel of a post. I'm really hoping someone will be able to help me out in getting this solved.

Cheers.

There should be something for a centering spring arrangement under that cap. If that broke or has come apart it could pile up and prevent the spool from going back to center. I would carefully remove the cap and see what is found. You may want to put a tarp down under it in case you drop anything. And you could get some oil dripping out. That would be my first check.

Jim has you going on the right track. I had much the same thing happen to a Champion grader, thankfully the location was great for access. I don't think you have to be too fearful of taking the cap off, in my case, if I remember right the spring in there was broken and made trouble for the valve moving. The clearance for pulling/sliding out the valve could be hard without removing the valve bank but you may not have to, the problem might get resolved with the valve part way out.
A similar problem happened on the valve bank on the Allis 745 loader I sold, they had a repair guy come in, I believe there was a piece of debris stuck in the valve like a piece of rubber perhaps from the inside of a hose... too many years have gone by to remember details.
If you have to loosen the valve bank, see if you can possibly leave as many hoses connected and still get enough clearance to do what you need to do. Some jobs just aren't pleasant to do, I don't like oil running everywhere, especially on myself, LOL.
It may not be as bad as you think, just take it one step at a time. Please let us know how you make out.

So, with everyone's advice on this forum and a couple different ones, I managed to get the spool out.

The first major breakthrough was figuring out that I can remove the seat, and then the bracket that the seat is attached to, which removes the back wall that was obstructing clear access to the valve block!







With that out of the way I could relatively easy get in and see what I'm doing.

I took the cap off the dipper spool and found what looks to be a pretty grungy screw.



I had to hook the linkage back up to the control level in order to be able to get the screw off without spinning the valve, but when I did get it off it was a spring assembly that was extremely dirty. And the spool itself still in the valve block wouldn't budge.

At this point I decided to take apart the spool next to it (bucket spool) to see if there was any difference between a 'working' spool and the faulty dipper spool. Sure enough, it was much cleaner looking (although still a bit grungy) and to my surprise, I was able to pull the entire thing out without taking the spring/screw off.

I reattached the screw to the dipper spool and tried pulling it, but no go at all. I ended up having to tap it out with a hammer and punch, which eventually worked, but holy was it ever stuck. At this point my guess, based on how the parts looked, is that water got int he system and was left too long, causing internal parts to rust and corrode to the point that the pitting and bubbling made it difficult for the thing to freely move back and forth. Even though it was working fine before, it was only a matter of time.

I ended up taking out every spool.



And then I decided to take off the entire valve block after pulling all the spools and disconnecting all the hoses.



At this point I'm going to take the whole thing apart and give it a good cleaning scrubbing.

What would be the best cleaner/solvent to use for this job?

Also, I'm left wondering what to do with the whole system at large. I'd hate to clean this thing up and then just throw it back on without doing anything with the rest of the system in case there's junk elsewhere that could get caught up. I need to take the pump apart probably and clean it too, as well as the diverter valve etc.

What would be the best way to flush everything out of the system after doing all that? I might as well do an FULL SERVICE on the hydraulic system after going this far!

Wow you've done a great job and good reporting too!! You may want to run a search for 'cleaning a hydraulic system' to decide which cleaning fluid/solvent to use and which to stay away from. If it was me I would not disassemble the pump unless it wouldn't be working properly. Find all the screens and filters, clean/change as applicable. If the hydraulic fluid is very milky looking it may have emulsified water in it. If you pull the plug at the bottom of the tank see if water comes out before the oil. Even if you change hyd. oil you still have so much of the oil left in the cylinders etc that it becomes only a partial oil change.
You've made very good progress, you can fix anything on that machine after this point!