The hydraulic fluid will follow the past of least resistance so if the packing is bad in one of the side cylinders the fluid will go there first before the free lift cylinder. On a lot of "foreign" trucks there are tubes at the top of each cylinder that return to the tank. If you take off the tubes raise the mast all the way up and keep hydraulic pressure on it once it has reached full extension there should be no fluid coming out of the tube ports. If there is then the cylinder in leaking internally and the must be repacked.