You could also spend $25 on a pressure gauge and plumb it in between pump and cylinder. It would help to narrow down the culprit.
Mike
This is the best advise so far. Repairing anything is best left to those that carefully diagnose a problem and dial in on the most likely suspect, and a pressure gauge will tell you exactly what component has failed. What ever you do, dont run the ram forward and keep it there in the vain hopes of diagnosing a failed cylinder seal. A hydralic system like a log splitter runs at almost 3000 psi, and if a seal has failed you can groove the narrow piston by continously running high pressure fluid across it. At that point you might as well junk the cylinder.
Start with the fluid. Is the tank full, and how does the fluid look?It should be bright, almost clear, and free from debris. Put some on a white piece of paper and look at the color. Next drop some on a dark piece of paper and with a bright light look for any metal fragments.If you have a filter inline, there shouldn't be any metal fragments at all.If its been awhile between filter changes, change it out. They are cheap to buy and you will need one anyway.
If you are unsure about the type of fitting and gauge you need, take the hose off between the pump and the valve, and also the hose between the valve and the cylinder. Take both with you and stop at a local hydraulic repair shop. There they will either have or will make up for you a fitting to allow the gauge to be placed between the fittings.
Start by placing the gauge between the pump and the valve, and see what the pump is putting out. You should have anywhere between 2500 and 3000 psi, depending on what the valve is set at. If you have less than 2300, you have either a problem with the pump or an obstruction in the line. If the engine is not bogging, it is very unlikely you have an obstruction in the line.
If the pressure is good between the pump and the valve, move the gauge between the valve and the cylinder.Again, with the engine running at peak rpm, the valve forward, and against a chunk of wood for a brief period of time, you should be at the same pressure that you had between the valve and the pump.If it is less, you have a problem with the valve. If its adjustable, adjust the nut to see if you can get any more pressure out of the valve.If its not, a good bath in solvent may fix your problem,or it may need to be either serviced with a kit or replaced.
If the pressure is good at all points, then all that is really left is the cylinder. Pull the piston out and examine the seals. Any sign of fluid passing by?Another test for a leaking cylinder seal is to place a chunk of wood on the splitter, run the ram foward until it hits, then stop the engine. Remove the return line from the cylinder back to the tank, and get a helper to hold a bucket in front of the open port.Start the engine and push the valve forward.Once the ram stops against the wood, you should have very little fluid trickle out.If you have a continous flow of fluid, you have fluid leaking past the seal and around the piston. You know at this point what to do.
Perform these tests and get back to us. We can help you from there.