Bed won't go up, can anyone help ?

[Rat Rod Mac]

I have a 1970 Chevy C-50 two ton dump truck that is mainly used on the farm. While using it this past week on about the third load I noticed the bed went up extra slow, ( when dumping I just let the engine idle and it lifts it up just fine ) the forth trip it would not go up at all. When I started in the morning I checked the hydraulic fluid level and it was ok, so I know it's not low on fluid. It's an old truck so I'm guessing it's the pump. I pulled the pump and the shafts turn free and easy, but a buddy of mine says that the wear plates could be shot and allowing fluid to slip past instead of pumping. Here's the problem, the pump has no I.D. tag on it and I really hate to go to the Chevy dealer for fear of them saying it's no longer available or else it will cost two grand to fix it. I just sent an e-mail to Surplus Center to see what they can tell me. So I thought I would bug you guys to see if anyone has had similar experiences. Do any of you guys know of a place I could get a hold of for a rebuild kit and possibly even buy one already gone over. Any and all suggestions would be greatly appreciated. Thank you very much. RRM

[Redneck in training]

Before you repalce the pump measure pressure it puts out. Or at least check if the cylinder or pump get hot. If it gets hot then is passing.

[texasjohn]

I had similar problem with hydraulic dump trailer once.

Problem turned out to be clogged filter in hydraulic system. Moisture had gotten into the system and mixed with fluid to make a white mayonnaise that stuck in the filter. Changing fluid did not solve problem.

I took apart the hydraulic tank and discovered (not obvious at all that it was there, not meant to be replaced regularly) an inline screen filter there where outflow goes to pump. I cleaned it and instant fix to vexing problem. I, too, thought pump was bad for a while.

I would strongly suggest you make sure all filters in your system are clear.

Hope this helps.

[Rat Rod Mac]

Thanks fellas. Two good pieces of advise there. My truck does not have a reservoir. I put fluid directly into the far end of the cylinder, so therefore I don't know if it has a filter or not. Just two lines from the cylinder to the pump and a power take off shaft to the pump and a directional rod that goes up thru the floorboard by the shifter, that's it. I did notice that when I took the first line apart the fluid did look like it had a milky color to it instead of the pure fluid tan color. Maybe I should try and drain as much fluid out of the cylinder as I can and see what happens. I've got twenty truck loads of dirt sitting in my field from the Township guys cleaning out ditches and I have to get it moved for spring planting. Thanks once again. It's appreciated. RRM

[Gary Fowler]

Are you sure you dont have a reservoir? Where does the oil go when the cylinder is lowered. All dump trucks I have ever encountered have a single action cylinder meaning the oil pump forces oil into the bottom to push the cylinder out, gravity lowers it. The fluid has to have someplace to go when the cylinder retracts. Even with a double action cylinder where oil pushes the cylinder in both directions, it would need to have a small reservoir in case of fluid lose and to keep the pump from cavitating when pumping while the return oil runs back.
You must have a reservoir somewhere and likely a suction filter somewhere also.

[Rat Rod Mac]

Gary, I'm no hydraulic guy, but I think this is how it works. The pump has two hoses attached to it. One for fluid exiting ( discharge - pressure ) and one coming back ( return - no pressure ). The cylinder has two hoses attached to it also, one at each end. Lets say the cylinder is laying on a bench East & West. The east end has the pressure line hooked up to it and in the middle of the cylinder is the piston with the rod attached to it and it is coming out of the west end. Now when the pump starts to run and fluid is pushing against the piston it is pushing it towards the west and the rod is coming out of that end. Now there is fluid between the piston and the end of the cylinder's west end. This fluid has to go somewhere, remember, you can't compress a liquid, so as the piston moves towards the west end the fluid is pushed out of the hose at that end and it goes right back to the pump thru the return hose to be run thru the pump and pushed once again to the piston. It's like a closed circuit, it just goes round and round. Now when it's time to lower the bed you reverse the pump valve and the weight of the bed pushes the fluid back thru the hoses the other way and the west end of the cylinder is now being filled with fluid. I believe this is how the system is set up on my truck. Thanks again for your response. RRM

[oldnslo]

RRM,
Unless you have a double rob cylinder with equal areas the oil flowing bath and forth through the pump will not work.

[Zork]

I don't think the oil goes round and round but I think he means more in a pendulum manner.

Lift: Pump discharge to cylinder input while cylinder reserve (output side) feeds pump via multi- way control valve.

Lower: Cylinder input dumps fluid into top of cylinder via gravity and control valve (pump bypassed)

If the fluid looks contaminated: Replace fluid.
Does the pump sound normal? Starved? Clogged screen at pump?
Is your 3way/4 way control valve (assumed) excessively hot compared to operation time and system temperature (valve may have malfunctioned causing short cycling of pump and heating of fluid.

Hope this gives you some ideas.

[pjbci]

Rat rod is correct. I have a small ford dump trk that uses the opposite end of the hyd cylinder as a resivoir.

[oldbones]

One other thing to check. My C65 tandem grain truck has a wire cable in a sheath (think of a lawn mower throttle cable, only heavy duty) to activate the directional valve on the pump- pull to raise, push to lower, center neutral. If yours is like this, the cable wire might have come loose on the pump lever. Have seen this many times. Just not sure what your is like.