Dump bed does not raise (loaded)

Sounds kinda normal but you need to know that pressure the pump is putting out at idle and then most likely ether replace the pump or pressure relief valve.

 

What ratio is the P.T.O.?

 

How much acceleration are you having to give it to get it to move?? When it does, is it still slow or does move quickly???

 

Sounds normal to me also. I have to get on the go pedal to raise my bed to.

 

With no prior knowledge of how the truck operated when new and what kind of hydraulic system/pump is on it the best I can guess is either the pressure relief is set below spec, dump box is loaded too heavy on front, or pump/control valve is starting to wear out.

Hydraulic systems use positive displacement pumps, which means they move the same amount of fluid every revolution. Contrary to popular belief, pumps do not create pressure. They create flow. Resistance to flow is where pressure comes from.

So we know that the pump is moving the same volume of fluid every revolution. If that flow is not raising the lift cylinder, that flow needs to be going somewhere. If the relief is set under spec or if the dump box is loaded too heavy, the resistance to flow is greater than the relief setting and all the oil dumps back to the reservoir. When the truck is revved up, more flow is generated, which could be more than what the relief valve can flow. System pressure rises and is enough to overcome the load on the lift cylinder.

Same as with a worn pump. Under light resistance (pressure), the pump is able to move fluid efficiently. As the pressure increases at the pump outlet, oil will want to take the path of least resistance. If there is excessive wear in the pump, oil will start bypassing the pumping elements and return to the inlet side (or dedicated case drain line, if equipped). The more load on a worn pump, the greater the drop in efficiency. Same story as above. If you rev it up, you produce more flow. The increased flow can't all leak past the pumping elements, so it makes the cylinder move.

Hydraulics are a specialized area of mechanic work. You need pressure gauges and flowmeters to properly test the components. Please don't go and crank down the relief valve or put a new pump on it based on what I wrote above. Take it to a place with experience on hydraulics because there are a lot of risks with hydraulic work (fluid spills, injection, stored pressure etc).

 

AModelCat is right, but what you have to understand is that your PTO(pump) produces enough pressure at idle to function the system. Add the weight of a full load and it requires more pressure to lift. More pressure means more flow requirement. More flow requirement means more rpm from pump.
If you check, Max system pressure should be proportional to Max capacity or close.

 

I own triable dump business and work on my trucks. Your pto is fine. Your pump is burned up. No big deal. No pump with air shift is 650 bucks. Non air shift which means cable operate dump lever is a few bucks cheaper.
Those pumps are not to exceed 1200rpm when pto is engaged. When you put a new pump on, you won’t need to use any throttle fully loaded. It will go right up and you will be rockin and rolling. New pump install take you about an hour. 4 bolts. When putting new one on, weighs about 80 pounds. Just roll it on to your chest and bench press it up into the pto. It will slide in and suspend its self with no pressure on shaft

 

That's exactly what I was getting at. Its a positive displacement pump so the oil HAS to go somewhere. Doesn't matter if engine is idling or if its revved up to 1200. Revving it up is just creating more flow to overcompensate for the internal leakage. If its not going into the cylinder to raise the box, its leaking internally somewhere. Either thru the pump, relief, control spools etc. I seem to recall most dump box cylinders are single line, so if oil is puking out the vent hose, could be one of the packings in the cylinder are on the way out.

Quick and dirty check: Components that are leaking oil internally will get very hot. So if one was so inclined, they could shoot various valves, pumps, cylinders etc with an IR temp gun. Components that are substantially hotter than the rest of the system are likely bypassing internally.

 

Spot on my friend.