this can be two different problems. First check the carriage roller clearance, make sure you do not have a binding problem. If this truck has a free lift cylinder, you need to check it for external damage to the barrel. If it looks good you need to ds assemble the cylinder and look for a defective check valve. I am not sure if Cat uses a check valve in the base of the ram like many other mfgs. This is the most common problem with Yale and Hyster trucks.