The cruise control on my '56 started acting weird after changing transmissions--The problem: after it was set, applying the brake wouldn't shut it off (I had to shut it off manually) and sometimes the turn signals (especially the left one)
would shut off the cruise. I checked for crossed wires, bad insulation, etc.??? Finally took it into a local cruise control specialist shop--no charge--bad brake light switch. Electrical bleed-back was the culprit relating to the turn signals shutting the cruise down.
