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Brake Booster Assist Ratio.

If your brake system has no booster and no ABS, enter a "1" (one) in the "Brake booster assist ratio" field.
If it has no booster but it has modern ABS acting as a booster, find out the assist ratio as if it has a booster.

If your brake system has a booster, enter the assist ratio. This is not a constant number, since the force applied by the booster does not increase linearly with the force on the brake pedal. Therefore, you are interested in the assist ratio at pedal pressure for maximum deceleration.You can find this number as follows:

1) Find a small bathroom scale* or other scale that can indicate at least 75 lb (34 kg) or 100 lb (45.5 kg), to show desired pressure on the brake pedal for maximum deceleration.

2) Find a spacer (piece of wood / two-by-four) or any other object that allows you to push the brake pedal with the bottom of the (bathroom) scale, using your hands (see illustration)..

3) Take off one of the front wheels to get access to the brake caliper and replace the bleeder screw with a pressure gauge designed for this purpose (see illustration). The best affordable gauge that I could find on the internet (it includes six adapters to six common bleeder ports) is from SSBC. Google it and find prices around $50 for the entire kit. Don't forget to bleed your brake caliper after you have reinstalled the bleeder screw.

4) With the engine not running, push the pedal several times to get rid of any vacuum still in the booster. Some boosters contain enough vacuum (or pressure, in case of a Hydro Boost) to assist you with 4 or 5 strokes of the pedal, so make sure all vacuum or pressure is gone. In case of a modern ABS with integrated pressure accumulator, it can take 30 strokes of the pedal (GM, Teves and Bosch) or even 50 strokes (Bendix) before the accumulator has been totally discharged and the brake pedal becomes harder to depress.

5) Get the spacer (piece of wood) and scale in position and, while you are sitting in the driver seat, press with your hands on the scale (do not squeeze, only push!), with the amount of force needed to have the dial indicate the number that you choose for "desired force on pedal for maximum deceleration". This will be somewhere around 75 lb. More or less is all good, depending on your personal preferences. Now with the scale indicating this number of your choice, have someone read the pressure gauge you installed previously in the bleed port of one of the front brake calipers.

6) Repeat step 5 with the engine running.

The assist ratio you are looking for is the gauge reading from step 6 divided by the gauge reading from step 5. For example: if step 6 produced a gauge reading of 1,100 psi, and step 5 a reading of 300 psi, your booster assist ratio is 1,100 / 300 = 3.667       (so in this case you would enter "3.667" in the "Brake booster assist ratio" field)

Note: You don't need to unhook the return spring that usually is installed between the brake pedal and the firewall or brake pedal bracket. This spring slightly affects your pedal pressure, but not the booster assist ratio, so for this measurement you do not need to disconnect it.

* Preferably a scale with mechanical dial, since most digital scales need to be at constant pressure for a few seconds before they display a reading, which is hard to accomplish when pushing 75 lbs with your hands.


Last Update: 04/12/2023
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