Many Thunderbird owners that are driving and enjoying their 1955/56/57 models on cross country trips are installing one of the modern electronic cruise control units now available for classic cars. These units work very well when properly installed. One unit made by Rostra Precision Controls, Inc. has became popular with Thunderbird owners. They come with good installations instructions, but I will say that installation is not for the faint hearted or inexperienced mechanic.
One very important item that makes these units function properly that is not part of the unit, but part of the car is the brake lamp switch. Our Thunderbirds were equipped with a hydraulic type switch originally mounted in a brake line brass fitting attached to the frame on the drivers side under the battery box. I would say that most early Thunderbirds are still equipped with an original type switch that more than likely has been changed several times in the last sixty plus years. The hydraulic type switch is effective, but as they age slightly more pressure is required to actuate the switch. In addition some of the hydraulic switches are made off shore and do not last as long as the originals made in the USA.
Disengagement of the cruise control occurs in two ways when the brakes are applied using the vehicles brake lamp switch and manually when non emergency disengagement is desired. We all know that occasionally a quick emergency stop is required especially during freeway traffic in which case instant disengagement of the cruise control is absolutely necessary. I know of at least one owner who experienced brake lamp switch failure with the cruise control engaged resulting in acceleration of the vehicle. Acceleration occurred when the brake lamp switch failed and the disengage signal was not sent to the cruise control causing the cruise control to detect the vehicle slowing as the brakes were applied. In this case acceleration occurred when the driver was attempting to stop. In an emergency situation the results could have been a disaster.
If you have cruise control or plan to install cruise control I highly recommend you install a mechanical type brake lamp switch. The mechanical switches mount on the brake pedal arm under the dash. The cruise control kits have instructions on how to connect to the mechanical brake lamp switch into the cruise control system. Major Thunderbird parts dealers have these switches listed in their parts catalog. Some are listed with the original type switch basic part number 13480M. The M suffix indicates mechanical.
Two types of mechanical switches are show, one is shown attached to the brake pedal arm under the dash. The one that is not installed is the latest version it also attaches to the brake pedal arm under the dash.