I have a 50 year old pedal that came with an antique machine I bought. It did the same thing. My electrician husband took the cover off and cleaned the electric contacts. Copper contacts will get a corrosive buildup on them that prevents proper current flow. A Q-tip dipped in alcohol should clean them to a nice shine and end the problem. If they are really bad you can use a pencil eraser and erase the buildup followed by the alcohol.