Re: Bendix Treadle Vac

Posted by HH56 On 2017/5/7 20:39:42

The problem with modern masters is they all push fluid ahead of the piston and on a power brake system require a pedal ratio of at least 3 and preferably 4 to 1. Most manual systems use around a 6:1 ratio. Pistons are generally short stroke, large bore and with the typical 1" master and approx 1" wheel cylinders, there is not much hydraulic advantage. The smaller master helps but still mostly relies on mechanical advantage.

The Treadlevac has no piston as such and fluid is moved out by the action of a solid cylinder of metal moving into an enclosed space and displacing the fluid which totally surrounds the solid metal. The stroke is longer and because the fluid is displaced rather than pushed, there is more hydraulic advantage and the pedal ratio can be less. In Packards case the power brake is at the end of the arm so the ratio is 1:1.

Several have tried to use the existing suspended pedal with the Packard location and it just cannot be done and have a safe brake system. Believe Craig was the first to keep the stock location with a modern power setup but had to modify the pedal to get a ratio within the recommended range. Others have moved the booster to the firewall vent area so it can be connected higher up on the suspended pedal arm to get the needed ratio.

Problem with modifying the pedal to get the ratio and keeping the master in the original spot is the pedal changes location by moving lower, sticks out farther, and mod is very obvious so no way to maintain the factory inside look.