410 B boom bleeding down

Hi,
Looking for some advice. I bought this backhoe yesterday and it’s in fairly decent shape other than the main boom bleeding down fairly quickly. You have to stay with it just to drive it around. The guy said he switched the hoses to another valve to check the cylinder and it stayed up so it must be the valve leaking by. Are these valves user serviceable and if so, does anybody have a parts diagram and parts list? Thanks.

I think I would reseal the cylinder before taking the control valve apart.
Yes the control valve is serviceable. It requires a spotless work area and a keen eye for flying parts.
You will be needing the service manual. Best money you can spend on the machine.
The parts catalog is here. ---->https://partscatalog.deere.com/jdrc/navigation/equipment/58084
The seal kit for the valve is here -->https://partscatalog.deere.com/jdrc/sidebyside/equipment/58084/referrer/navigation/pgId/166935

Interesting info for your machine here --->
https://www.deere.com/assets/pdfs/common/products/backhoes/Backhoe-410B.pdf

Thanks for the info

Ok so I got the service manual and the tech manual, which obviously is way above my head on many levels but still good to help me understand what makes things work and why. I have studied these heavily, too bad they don't have a troubleshooting section. I ordered a spool kit for the boom section of the control valve and boots for the other 3. Finally yesterday I was able to pull the control valve assembly. I then pulled from the boom section the spool, relief valves and lift check and to be quite honest, everything looked pretty decent I was expecting to find a broken spring, wadded up o ring , pitting, trash or ssomething but no it couldn't be that obvious. Let me recap the problem and what I've done so far. Everything works great and all cylinders seem to have good power both directions. After I raise the main boom and release the handle the boom leaks down fast. Fast like in one minute or less. If I apply pressure ever so slightly on the valve control handle it stays up fine. I rebuilt the cylinder first thing so it's good. The check valve looks good and moves freely. It doesnt seem to have much spring pressure but I pulled out one of the others that works and it was the same. I'm wondering if it's one of the circuit relief valves causing this leak by? Since it's the boom staying up that I have the problem with would it be the boom up relief [first obvious thought] or would it actually be the boom down relief instead? Thanks

Update: Disassembled the boom up circuit relief valve. There's a cone shaped needle valve that goes into a squared edge seat. the seat looks good but the cone has a pretty substantial groove wore all the way around. I'm assuming that's my problem. Other problem is that John deere only has available a o-ring seal kit and the complete new valve is out of the question according to them . The complete valve is AT74133. Maybe there's a aftermarket available? I looked at some of the cones in the other valves and they are in much better condition some with little to hardly any noticeable wear. If I can't find anything I've got half a notion to swap cones with the curl out relief valve and call it a day.