If the pallet truck drops down immediately after being lifted up, or it lifts without load but won't lift with load:
Take out the part in video above, press the handle several times to let out some oil and impurities and put back the parts in original sequence to fix the issue.
Generally cleaning the hydraulic valve as shown in video will solve problem. If not, need to replace the whole hydraulic valve assembly (not just the screw taken out in the video).