The only necessary jobs to keep these going is replacing the gaskets and inspecting the 1/4" rods that hold it all together for dezincification. If you see pink, you have a problem.
Older units use cork gaskets, new units use rubber. Make sure all the old gasket is removed.
I like to do a full disassembly and clean away all the crud. The lens can be buffed inside and out with a cloth buff and compound.