Twin Chamber Damping Rod Seals must be perfect. If the cartridge is low on oil upon disassembly this seal may be leaking (check the Damping Rod Seal and Reservoir Piston O-ring as well). Also check the shaft itself for wear and replace as necessary.
Note: early Showas with 12mm damping rods have an extremely sharp edge at the thread relief. This must be carefully deburred with a file before disassembly.