D8H blade won't raise

[MKing154]

I have a 1965 Caterpillar D8H, S/N 46A10615.
I bought it earlier this summer for some road building projects around my property and finally got around to putting it to work today. A buddy was operating while I was on another machine, and he had to stop when he noticed the blade would not raise as it should. Upon stopping he noticed a steady dripping leak from a coupling on the tilt cylinder hoses, near where they come out through the grill. We tightened those up and the leak stopped, and we decided it would be best to check and refill the hydraulic oil tank as needed. After doing this, I checked the controls and the blade still would not raise. We traced the control rods from the tiller-style blade height control, through a 90° redirect, and to where the rod operated what looked to me to be some sort of valve control on the back side of the reservoir. Everything looked to be connected and operating as it should (to my novice eyes anyway)

The lift cylinders will still push the blade down powerfully enough to lift the front of the dozer off the ground, but they will only allow the blade to raise back up enough to settle the front of the machine back down, and they will not lift the blade clear of the ground as they had been doing all day prior to this happening. It's almost like the blade is in some kind of float mode. Do I need to somehow bleed air from the cylinders or the valves on/in the back of the reservoir?

I'll attach some pictures labeled with my non-expert lingo.

Thank you in advance for any helpful suggestions.

 

[Olfolks]

I think there is a over pressure relief valve built into the cat Hyd cylinders, these can cause a symptom such as you describe when they start to leak internally.

 

[MKing154]

I think there is a over pressure relief valve built into the cat Hyd cylinders, these can cause a symptom such as you describe when they start to leak internally.

You don't suppose such valves would be something that would have to be bled manually then, would they? My very basic understanding is that most hydraulic systems are self-bleeding/priming and any air that has gotten into the loop will eventually make it's way back to the fluid's reservoir. I suppose if the valve(s) you speak of were leaking because they were worn out, bleeding air out of the system would be a moot point.

At any rate, thank you for this suggestion, I'll see if I can't read up a little more about these valves. I'm thinking I might have to spring for the full maintenance and parts manuals, instead of just the operators manual which has thus far been my only reference literature.

 

[TomA in CA]

I think there is a manual for the hydraulic system and one for the blade. You may need both.

 

[AJ.]

As I recall the cylinders of those machines had two seals on the piston, one for the down pressure side and one for the raise side, if either of the raise side seals has let go the blade will not lift, you need a mechanic that can test the cylinders for you, I think it is highly unlikely that the trouble is in the valve, the seals in the cylinders are wearing parts that have to be replaced now and then.
AJ

 

[MKing154]

As I recall the cylinders of those machines had two seals on the piston, one for the down pressure side and one for the raise side, if either of the raise side seals has let go the blade will not lift, you need a mechanic that can test the cylinders for you, I think it is highly unlikely that the trouble is in the valve, the seals in the cylinders are wearing parts that have to be replaced now and then.
AJ

Yes, the fella I bought the dozer from said he had repacked the lift cylinders within the past year or so. I just had the tilt cylinder professionally repacked before the machine was hauled to where I've got it now.

Unless the previous owner did a hack-job and only replaced half of the seals? ...But that wouldn't make sense because the wiper seal looks clean and almost brand new, and in my limited repacking experience, you've gotta remove the end nut or bolt and get past the metal ring that holds some of the seals (the raise and lower seals) at the far end of the shift that rides inside the cylinder.

 

[mjsnodgrass]

Yes, the fella I bought the dozer from said he had repacked the lift cylinders within the past year or so. I just had the tilt cylinder professionally repacked before the machine was hauled to where I've got it now.

Unless the previous owner did a hack-job and only replaced half of the seals? ...But that wouldn't make sense because the wiper seal looks clean and almost brand new, and in my limited repacking experience, you've gotta remove the end nut or bolt and get past the metal ring that holds some of the seals (the raise and lower seals) at the far end of the shift that rides inside the cylinder.

i would think if one of the lift cylinders had blown the other cylinder would lift the blade. i think your issue is elsewhere. good luck

 

[AJ.]

If the seal on one cylinder has failed, the blade will not lift as the cylinders are plumbed parallel, with the machine jacking itself up it says the pump is ok, the main regulator valve is ok, it is possible the relief valve for that circuit has an issue, the problem to inspect the valve involves splitting the tank, after sixty years working repairing Cats of all sizes apart from the control shafts coming out of the tank leaking I can't recall any other trouble from the valves, what I would do now is find the line that is feeding the lift and blank it off, gently pull the lever and see if the engine starts to bog down, if it does not then there is an issue in the valve, if it does bog down then the issue is in one of the cylinders
Good luck
AJ