safety check valve in freelift cylinder most likely
pull out ram and check the valve at the base of ram.
could be sticking
if valve is missing you will need to get magnet and fish out the parts in bottom of tube, order a new valve and snapring (usually in a safety valve kit).
clean out all metal particles
go to return filter in hydraulic tank and check, change filter if metal found and flush out return line.
Pull velocity fuse in primary cylinder on mast, (usually on right side where main line comes in and check for metal particles, if any found change valve and flush lines.

Most of the time i've found that the valve and any metal debris is usually contained in the cylinder tube at the bottom and rarely makes its way past that point.