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The main reason newer cars have power brakes is ease of drivability. So even grandma can push the brake pedal down and stop safely. If you remove the vacuum assist to one of those systems (engine off) and try to hit the brakes it's suddenly very difficult to stop the car because the pedal effort went through the roof. The reason for that is most power brake systems have a very low pedal to master cylinder push rod ratio, like 2:1 or even 1:1. On cars without power brakes mechanical leverage is used to reduce pedal pressure but maintain easy braking. Cars with manual brakes use a pedal ratio as high as 7:1 and in the beetles I believe it's about 6:1. So 100lbs of pedal pressure in a car with power brakes is about 200lbs of force at best without vacuum assist and poor grandma will break her leg before she stops the car. With manual brakes 100lbs of pedal pressure is good for 700lbs of pressure at the master cylinder and even grandma can hit the binders hard in a properly setup manual system.
Power brakes in a beetle isn't worth the extra hassle and the money would be better spent on a manual setup. However, there are mechanical brake boosters that reduce the pedal force a bit which could be a better and cheaper option.
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