A pallet jack that does not lift has air inside the hydraulic system. The reason air gets inside is often due to a bad O-ring at the valve cartridge. An inspection of the hydraulic system near the bottom of the handle often shows signs of a fluid leak. To fix a pallet jack that won't lift requires replacing the O-ring and refilling the hydraulic system with fluid. Replacement O-rings are available at tool supply centers and from the manufacturer of your pallet jack. In most cases, you need the model number of your pallet jack to order the correct replacement O-ring.
Place the pallet jack on four unextended jack stands, two on each fork. Ask a friend to assist you in lifting the jack and maneuvering the jacks stands under the forks. The jack stands should be at each end of both forks.
Locate the oil drain screw on the right side of the hydraulic pump body beside the rear tire. Place a container under the stem and remove the screw with an Allen wrench. Pump the pallet jack handle up and down until all the fluid drains. If necessary, have your helper hold the front of the pallet jack forks to prevent the jack from tipping as you pump the handle. Place the drain screw back into the stem securely.
Find the pin that secures the lower lever to the right side of the pump body. Place a small Phillips-head screwdriver over the end of the pin and tap the screwdriver with a hammer to drive the pin out of the lever. Pull the lever away from the pump body.
Unscrew the valve cartridge that the lowering lever covers on the hydraulic system. Use a pair of pliers to start turning the valve cartridge counterclockwise until you can unscrew it by hand. Remove the O-ring from the ridge in the pump body where the valve cartridge sits. Wipe the area clean with a rag to remove any loose debris left behind by the old O-ring.
Place a new O-ring into the ridge and screw the valve cartridge back into the pump body. Tighten the valve cartridge until you can no longer turn it by hand. Turn the cartridge another 1/2 turn with the pliers.
Position the lowering lever back into the side of the pump body and secure with the retaining pin. Remove the screw on top of the pump body furthest from the pallet jack handle. The screw may be a Phillips-head or Allen head. Pour hydraulic fluid into the hole until the fluid reaches the bottom of the threads in the hole. Place the screw back into the pump body and secure with the appropriate driver.
Lift the pallet jack off the jack stands with the assistance from your helper.