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This video shows how to check your log splitter for a bad pump.
If you aren't getting enough pressure to split wood, follow these steps in the video.

A quick note about the hydraulic pump. The pump doesn't actually create pressure. The pump is comprised of gears that when the pump shaft rotates at a given rpms, fluid exits at a given flow determined by the gear set. The pressure is created by basically restricting the fluid flow from the pump, i.e. the valve.

1) Most important step is to check that you have both the coupler keys properly in place. The most common issue we find is that when you assemble the pump to engine the keys can sometimes fall out during installation. Without the keys in place the log splitter is still able to rotate the engine and pump shafts due to friction. It's not uncommon to still be able to get pressure over 1000 psi without the shaft keys in place.

2) After you have physically checked that your coupler keys are present, the next step is to see if fluid is flowing from the pump when the pump shaft is turned. There are several ways to check this, we could remove the pressure hose going from the pump to valve and put the hose into a bucket. Pulling the recoil of the engine will slowly rotate the pump shaft which will in turn rotate the pump gears and force fluid from the discharge port of the pump. This can be kind of messy.
A cleaner method would be to put the valve in the neutral position where fluid from the pump will go straight through the valve to tank with minimal resistance. The engine recoil should be very easy to pull because the pump will have no resistance as fluid is directly going to the tank.
If you place the valve in reverse (detent position), the valve is now directing fluid to retract the cylinder. The valve is directing fluid and creating a restriction in the fluid flow. If you were to pull the engine recoil it will be a lot more difficult. The pump now has to overcome pressure in order to rotate which can be felt by the increased resistance in pulling the engine recoil.

3) A pressure gauge and shutoff valve can diagnose the pump more accurately. Connect the pressure gauge between the pump and the inlet of the valve with a shutoff valve on the downstream side of the gauge. Similar to diagnostic test #2, pull the recoil with the valve open to get a feel what no flow restriction feels like. Close the valve and pull the engine recoil and watch the pressure gauge build up pressure as the flow from the pump is 100% restricted. If your pump is functioning properly you will see an increase in pressure on the gauge when the valve is shut.

4) You can perform the diagnostic test with the engine running but you will need to take precaution. Make sure to have fittings and a pressure gauge that can handle the pressure of your log splitter.

Turn on your engine with the log splitter valve in the neutral position. Close the shutoff valve and watch as the pressure soars and the engine begin to struggle. Open the shutoff valve turn off the engine. The pressure gauge should have recorded the max pressure the pump can overcome given the engine size.

Do you need a shutoff valve or can you just use a pressure gauge and deadhead the cylinder?
Deadheading the cylinder is referred to by extending the cylinder all the way and holding the valve in that position. What happens is the cylinder is unable to extend further causing a fluid flow restriction. The fluid flow restriction can now be seen on the pressure gauge as pressure will build up fast.
Yes, this will work to show a properly working pump but only if your other components are functioning properly. For example, if you cylinder has a bad piston seal, when you deadhead the cylinder the fluid flow is not 100% restricted. If the cylinder piston seal is bad fluid can still pass. The cylinder can be functioning properly but you may not get a reading on the pressure gauge as pressure is not building up since fluid is passing through the piston seals.
Deadheading the cylinder also doesn’t account for the bypass setting in the valve. The valve has a bypass setting to allow the fluid resistance to be properly managed. If your valve bypass is set for 2500 psi, you will never see a 4000 psi pressure in the cylinder.

In conclusion, the best method to properly diagnose your log splitter pump is to use a pressure gauge with shutoff valve.