The switch is simple and robust. The brass contacts can wear and the paper insulators can go bad. The switch is available from lots of vendors and only 2 screws hold it to the starter. As you have discovered the "Bendix" is mechanical. If you can engage it it works. "Sometimes" working is probably the switch, and the contacts. New one is not expensive: http://www.classicpartsusa.com/product/EL102_1952/1952_Truck_Parts_Electrical
Just one vendor, there are others.


Steve H