Grain beds do not have a rear tail gate, attached at the top of the bed, like a rock/gravel bed.
Tailgate does not swing, only has a small gate in the center of it, that opens manually. It has to be a controlled opening (by hand) to prevent grain spill when dumping into a pit at a grain elevator/storage site.

You need to find that check valve on, or in, the pressure pump, before it blows a seal or hose.

Is your pressure pump inside a big, square looking tank? PTO shaft from transmission runs to the pump.
Most all older grain beds had that style of pump.

 

Well, took a good look at it today. I found out the cable went to the rear hinge area just about 8" above the center line of the pivot, thru a hole and looped back on its self then clamped with a cable clamp. It was set up so that once the bed reached the top of its travel, the cable will have pulled the valve closed or at least off of the raise position. Crude but effective. Also found that yes the square tank is above the valving and pump which is driven from the PTO. Started to clean up the area after blocking up the bed, as was suggested, for safety, thanks for that input. Kind of scary looking at the huge bed sitting up there, so I blocked it from the ground with a 2x8 to the hinge pin and blocks lying tight to the hinge area, just in case I rolled the truck some. I did find a tag mounted to the top of the tank mounting lip, but could not make out anything at all on it. I am thinking the cable was a farm fix for a stuck or broken relief valve. The cylinders have threaded collars, with set screws to hold them in place on the packing end. Is this packing available, is it a chevron type, are the seals split so they can be replaced without pulling the end of the cylinder rod apart? I can see that I need to tighten the collars a bit, or replace the packing. While I don't see a leak, I think due to the amount of dirt collected on and in the area tells me oil is leaking out somehow in the area. The heaviest dirt area is the cylinders. The tank is about 17" deep, I had about 3" of oil left in the tank once the bed was up.

Thanks again for bringing me up to speed on this.

 

Ok. Cable was someones idea of a fix, or a handy way to stop the bed if they were back there by themselves.
I would remove that.

***With bed down, the tank should be close to full. Do not add more if the bed raises ok. If you add to much, when you let the bed down, you will have one tremendous **oil slick** when that too full tank pukes the oil out the vent cap.**

Packing on the cylinders. If there is no obvious drip on the floor from the cylinders, they are ok. The scraper seal inside that keeps the shaft clean of oil when it comes out, probably has some wear. No drip, no fixie.

All hydraulic cylinder seals should be available. Find a shop that does hydraulic work on dump beds, hay forks, tractor lifts, etc.

 

Oh. If you do need oil for the tank, buy universal hydraulic fluid, available at farm supply store, and also even walmart carries it.

 

So, I left the bed up all day and did get a small puddle under the pass. side cylinder. It seems to leak also while lowering the bed. Not a big leak but one that allows oil to spread onto the outer part of the cyl. then it will collect fresh dirt where I removed it all. Can you shed anymore info on the hydraulic pumps they used on the trucks. It appears to be like you suggested all the workings under the square tank, driven by the PTO.

 

See if there is a set screw on the ring/head that screws onto that cylinder. If there is, you may be able to snug the ring up. Not too tight though if it has packing in it. It may just have a o-ring.

Does the pump/tank look like this? Have to view through the not available sticker there.
https://www.heavytruckparts.net/deta...821960&store=1

Mine looks like that one, but I currently have a hose broke on mine or I would raise the bed and pic it.

 

No, my tank is almost a cube to look at it. I will look again but I think my pump and valve are mounted on the bottom of the tank. Oh, yes there looks to be a set screw on the collars, I will loosen that one up and see if I can give it a little squeeze.
At least I'm still having fun.

 

Ok. There are different variations of those tanks. Thats the only pic I could fins on google.
Mine is mostly square, and the pump (as most of these are) is *in* the tank.