soft brakes - why?

[Bob Mulry]

Originally Posted by nhramnl

I learned this from Pustelny and it works every time. Fill the master cylinder, open the bleeders at all four corners and just let the car sit for a few hours. Obviously, check the master cylinder regularly, to make sure it doesn't go dry. The system will gravity-bleed just fine. To make absolutely sure you're done (though it is not necessary), close all four bleeders and put one pump on the brake pedal at each corner. You should get solid fluid, with no sputtering.

One other thing. the basic rule of master cylinder bore size says that too large a bore will make the pedal go too deep and the brakes will tend to feel spongy and too small a bore will make the pedal too hard and require a ton of pedal pressure.

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Good info except you got it backwards regarding the ratio of master cylinder size vs. wheel cylinder / caliper size.

The smaller the bore on the M/C the more PSI you generate within your brake system with all things equal. Now you with a small bore M/C you have HIGH PSI and LOW VOLUMN per pedal stroke.

With a large bore M/C, you have LOW PSI and HIGH VOLUMN.

Due to the large caliper bore size a small M/C might not apply the brakes fully with 1 (one) pump of the brake pedal.

A small bore M/C will feel softer than a large bore M/C due to the mechanical advantage of the small bore = higher line pressure.

A large bore M/C generates lower internal pressure and the pedal will feel harder because of the lower mechanical ratio.

Don't forget there is also a pedal ratio that can be used to your advantage but the same mechanical rules apply.......high multiplication = less M/C stroke...lower multiplication = more M/C stroke.

Just make sure that you have enough volumn to fill the calipers / wheel cylinders with 1 (one) pedal stroke.

With a high muliplication pedal ratio, a small bore M/C and large wheel cylinders we used to bend the backing plates in half prior to the use of trans brakes.......


Hope this helps,
Bob