5000lbs lift truck 3 stage mast. Free lift/carriage is lowering at twice the speed of mast channels/secondary cylinders.

Mast raises normal and has no problem lifting a load. When lowering from any height above free lift. The free lift will lower 1st and faster then the channels. If you release the handle to the neutral position the channels will continue lower and push Freelift up until the pressure is equalized. I have lubed the channels, have taken apart Freelift cylinder and resealed. Secondary lift cylinders have clear vent hoses at the top and there is no oil in the vents. At a loose as to what is causing this problem. I asked the customer if this problem just started with no warning or if the problem came on slowly....replied that problem just started out of the blue. I also chained up the carriage and lowered mast to see if the speed of the channels increased...only slightly faster.
Removed and disassembled a flow control valve that is in-line with supply line. Saw no obvious problems with valve.