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Two types of combination valves are shown in picture 1. When installing a combination valve follow the suppliers instructions. Also shown in picture 2 is a residual pressure valve installed to the left in the rear brake line at the existing brass block.
Several companies are offering after market front disc brake kits for the 1955/56/57 Thunderbirds. Disc brakes when properly installed will outperform the original drum brakes. The performance is especially noticeable while stopping in a straight line; pull from side to side is eliminated.
Disc brakes became an option on some Ford vehicles from the factory in the early sixties. FoMoCo provided a proportioning system on all their vehicles as part of the overall package. The proportioning valve soon evolved into a combination valve which provided proportioned pressure to the rear brakes and metered pressure to the front disc brakes. The basic conversion kits for the Thunderbirds do not contain a combination valve.
Disc brakes without a proportioning/metering system will work.
However, in most cases the front brakes do almost 100% of the work in normal stop and go traffic even with the rear wheel cylinders reduced in size. In mountainous terrain during long down hill descent where the brakes are frequently tapped to keep the speed under control an overheat condition may occur on the front disc brakes. This overheat condition occurs because the rear shoes do not engage under light pedal pressure unless a combination valve and residual pressure valve is installed in the system. It is easy to determine if a combination valve is necessary when you convert to disc brakes. Jack your car up with all four wheels free to turn, press on the brake pedal until the front wheels lock if the rear brakes are not engaged keep pressing the pedal while turning a rear wheel until it locks, note the amount of pedal pressure required to lock the rear brakes compared to normal pedal pressure for stop and go driving.
The basic disc brake kits can be greatly enhanced by installing a combination valve (proportioning & metering valve). In addition a 10 lb residual pressure valve can be installed in the fluid line to rear brakes. A residual pressure valve is a one way check valve that maintains enough pressure on the rear brakes to keep the shoes close to the drums without drag resulting in a firmer pedal.
For more details on the combination and residual valves go to www.mpbrakes.com click on the technical section at the top of the home page then click on References (FAQ), then click on valves. Also read all the information on disc brakes, master cylinders and power brakes.