Komatsu PC250 slow bucket dump

So, as the title says, slow bucket dump , somewhat random, sometimes better when cold.
All other functions normal and correct time.

Owner had some guy fix it but this owner never gives us the whole story, and its now back to doing it.
We have checked all pressure switches and sensors, all good.
Anything come to mind with you experts?
thanks in advance.

Bound up pin bushing
Bent linkage
Bent rod
Nut loose on rod allowing piston to bind in tube

Not an obstruction in the line causing low flow?

Good day
Suggest work from the cylinder - check for by-pass of fluid - make sure that the spool is full shifting - test pressure on the dumping line compare with the opposing direction that should show up a leaking shock valve in addition to those suggestions made.
Kind regards
Uffex

Full model number and a serial number is necessary. Have you checked the basics yet? Any visible damage to the cylinder? Have you pulled the return filter looking for plastic material or steel? How many hours on this machine? Any leakage on the cylinder? Any fault codes on the monitor panel?

A fast and easy check, is switch your pilot lines on the control valve. (Opening/closing), if the problem is on the opening side then, remove and check spool valve, if no changes, then problem is your cylinder.

All mechanical things have been checked, it is narrowed down to the spool or pilot. Its like it doesnt send enough pressure randomly to stroke the spool. The owner is one of those impatient guys that you never get the whole story from, then when we have time to work on it he "needs it" and we cant get far on it. So he can deal with it.
If we do get time to solve it I will post.

So we were able to get back to this issue.
PC250LC-6LE
A83030

The dealer cannot find a parts diagram of this main valve that has the correct o-rings even.
Entire machine work great except bucket dump is slow.
All the obvious basics have been covered
To clarify, my coworker and myself have 40+ years each working on equipment, mostly JD.
Spool has been removed and is free in bore, valves in that circuit are newish and clean, no metal or scarring.
Pilot pressure , modulates up to 500.
Here is the testing, Dig is curl up, dump is curl out.
Hanging with a bucket on, dig takes 1900 psi measured at pump to start moving,dump 2900.
Dan switched hoses at the boom. dig ,1000 to start moving, dump 4400
Installed test blocks at boom hoses, dump motion, 2000 psi back pressure when it should be near zero, dig no back pressure as it should be.
Wrong spool? in upside down?
And what the hell is the two pistons on the bottom of the spool in the housing, looks like some sort of limiter/ modulator, and these are now dumping to tank to make sure spool full strokes.
Thanks
Russ

The A serial number means the machine was made for here. The dealer should have no problem getting info from the factory on that machine. Have you opened up the cylinder to see what is inside? Does this machine have a three way return on the bucket circuit? Do you have a photo of the where the two pistons are installed? I've seen lots of issues with the boom and stick before. Never seen your type of problem with the bucket circuit.

Its not the cylinder. other " mechanics" needlessly had it rebuilt..
What is under the long cover between the work ports? Schematic looks like double check valves.

John, you in portland by chance?

I'm between Tacoma and Seattle. I'll get into my old books later today. In those years Komatsu was constantly changing whole valves and control circuits so I'll see what I have that might cover your machine.

That would be great, Im thinking we should have just removed the modulator pistons from it while in there. This machine is going to finish its life in a quarry so no need for grading feathering.LOL
We have a service manual but its pretty worthless.
And im not good enough with hydraulic schematics to decipher operation.

Your serial number info shows the machine to be a 1997 model so you likely have the possibility of lots of other issues as well. Does the bucket function change when you operate other things as well? Try swinging at the same time, traveling at the same time or boom and stick together with the bucket at the same time and let us know what happens.
The book I have covers early 8000 serial number machines but I think it should be close enough. The two valves or spools on the outlet end are sub unloaders and I don't think have anything to do with your problem. In looking at the schematic, it appears that one of the compensator valves on the bucket function is leaking sensing oil thru and preventing the opposite compensator from opening. The pressure compensators are just load check valves that have signal oil coming from all the other functions which apply the same hydraulic pressure to hold the check valve closed as that other function is at. It's done to allow multiple functions with different operating pressures to operate all at the same time. Say you are operating the boom up and the bucket close at the same time, boom up pressure might be 2,000 PSI while the bucket could be closing at 3,500 PSI. The load check would have the 2,000 PSI holding it closed until main pressure on that function was able to equalize pressure. If that function wasn't there, the bucket close function would operate at the same 2,000 PSI as the boom.
The pressure compensators can be resealed but the big rub is recognizing when there is enough wear on the components of the valve that new seals would not help. When the first problems started happening back in the day, the dealer just replaced all the compensators on all the individual valves. That was expensive and involved replacing all the hydraulic oil as well.
Were it me, I would pull those two compensators out and tear them down to inspect and go from there.

We have done all the multi functions, bucket never changes.
The dealer did exactly what you are saying, changed all the compensators, I do not know why. I am told this problem was new.
The compensators we pulled look new still. No wear or scratches, no debris.

I studied the schematic and still dont see why it would have back pressure on the backside of cylinder in dump function, doesnt the spool just dump to "tank" cavity during this?
Thanks.

What doesn't make sense is that it only happens on the bucket circuit. It's the last function on that valve and that oil should all go back to the cooler, return filter and then the tank. The schematic did show the return going through something else before it got back to the return side of the spool. I'll do some more studying and get back.

I've looked again and the only thing that strikes me is that maybe the bucket spool got flipped end for end. You might try popping it out and flipping it over and see what happens.

It has the modulator pistons on the bottom, I personally didnt see it, can they be put on the other end of the spool.
What about the shuttle check valves between the work ports.?

I'm not sure of what you are calling modulator pistons. The load sense system doesn't need any modulators to make everything work smoothly.

The check valves are LS checks and they are part of the load sense system as far as I know.

There are 2 "rings" on the bottom of the spool, they are inside a longer than normal pilot housing . I have no idea what they are called, I think I sent pics earlier.

Those are spacers put in to center the spool in the correct relation to the ports in the valve body. My book doesn't show them specifically but the pictorial schematic does show that side of the spool being on the bucket open side of the valve body. My book shows the end with the extra body should be on the top side of the valve body. There are a couple of little balls on the inside of the valve spool as well. When you had the spool out, did you check that those were there and not stuck to one side or the other?