I have a JLG- Sizer lift that when I raise the primary boom it quickly lowers to the stored position. Is this a cylinder problem or a control problem?
Yes. Could easily be either. If you're handy, try installing a ball valve in the pressure line from the control valve to the cylinder wherever you have handy access, raise the boom, close ball valve. If it stays up, control valve issue, if it falls down, cylinder seals.
cyclinder is bypassing internally need to remove it and repack, replace counter balance vavles in the thing while your at it