M817 Won't Dump???


Waurika, Oklahoma
I just recently purchased two M817 5-Ton dump trucks. One is in excellent condition - the other probably a parts truck. Anyway, I'm having difficulty getting the dump to raise. I have checked and adjusted all of the external linkages from both the dump lever as well as the transmission driven pto box and hydraulic pump control box.

The bed will dump when empty (though you have to rev the engine up a little) but it will not raise even a light load. Also, I noticed that it will not "hold" and seems to just bleed down as soon as you move the control lever from "power up" to "hold". The fluid reservor is full and everything looks (externally) to be in great shape - no leaks, etc.. The control rod that goes up and down in the control valve body appears to be moving a full range of motion.

I have read the applicable manual pages and also searched the forums but still not sure what to do. I am suspicious that the control valve has something wrong in it and it's letting fluid bleed around the valve and, in effect, bypassing internally instead of sending adequate pressurized fluid into the hoses and cylinders. But, in looking at the exploded view of the control valve it doesn't appear that the valve rod (spool they call it, I think) has any replaceable or wear components like seals or O-rings?

I'm hoping that someone here has some experience with this. I could swap the assembly from my parts truck or I could take it off and just do some exporatory surgery and see if the bore or spool is worn or damaged but wish someone that knows more about it than me would lend their advice.

Thank You


Steel Soldiers Supporter
S.E. MI.
I would also check the fluid for water, is it milky? if so it could have froze and caused some issues with the pump and control . how is the filter, does the pump sound winey when engaged, could be starving for oil.


Well-known member
Mesa, AZ
Had a similar problem here with body not going up and determined trunion that raises spool in control valve was not moving enough to raise spool to route
fluid to cylinder. If you pushed it up from below with your finger it would work. Lengthened one of the rods by turning clevis out and problem solved.


Steel Soldiers Supporter
You're right about the spool not having any seals. They are a machined fit and when the wear out the only fix is to replace the whole valve ( spools wearing out are rare though).

It could also be that the cylinder packing is shot and letting the fluid bypass enternally. You've definitely have fluid bypassing somewhere. Could be the pump worn out, relief valve out of adjustment or broke, or a bad cylinder packing.

I'd start by removing the hose going to the cylinder and attach a pressure gauge and see if the pressure is within spec at that point. If you have good pressure there your cylinder needs to be rebuilt.

Next make for sure you're getting full movement of your control valve.

If your pressure is low there the next thing I'd check is the relief valve. I'm not familiar with the 5-ton dump system but if it's like most, the relief valve is servicable and adjustable. Take it apart and make sure you don't have a broken spring or any pitting in the sealing surfaces. If that checks out ok adjust it up (if it is adjustable, if it's not you can shim the spring a little with a washer) and see if that raises the pressure on your gauge. If the pressure is still low put the relief valve back to it's original setting and look further.

If all of that checks out ok, you're pump and or control valve is worn out.

Note- all this is assuming you've already checked to make sure you have plenty of oil and any filters have been replaced.


New member
Vancouver/Yakima, Wa
one of mine did the same, so i just had someone i trusted with my life (father :) ) and he moved controls while i laid under to see what was going on, for some reason the linkage got off and way out of 'wack' when just days before it worked normal... with some tinkering of the linkage and testing manually by putting it in gear and out of gear by hand we got her figured out... just my 2cents of help, may help or may not :) either way good luck! ps the others ahead of me had some good ideas/leads too....!


4 Star General /Moderator
Staff member
Super Moderator
Steel Soldiers Supporter
If you do anything with the dump body up or partially up, use some sort of safety mechanism(wood cribbing, steel pipe or something strong) to insure that the dump can not come back down and injure you.

Every year, many people are killed by dump truck boxes coming down on them. Please be very carefully and think several moves ahead of what you are about to do.


New member
United States
My M-817 hoist was acting up in that I could not get the pto engaged to the pump. It had always been touchy but became worse so I disconnected the linkage and manulaly twisted the pto shaft and saw the distance required to get engagment. The linkage box was not allowing enough travel, Hmm. So I took the box apart and found the bushing Part # 3 in the parts drawing was missing. Part 3 slides into the cam arm, the bushing was 3/8ths" ID, 5/ths" OD by 1/2" in length. Put a bushing in ,that size and problem solved. A bit easier to pull the cotter pins out of the lingage pins so that you do not mess with the adjustment and you can then get an idea as to what is going on. Nice thing is you can work on these beast!
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