Why would changing to a larger brake master cylinder decrease braking pressure?

[Lugnuts]

Hi,

I am in the process of restoring the brakes a 1974 Porsche 914 and upon reading up on a Porsche forum found a discussion about changing the original 17mm brake master cylinder with a 19mm one. A number of individuals indicated that doing so would reduce braking pressure and I was unclear as to why that would be.
It would seem to me that with all brake lines and calipers staying the same that a 19mm master cylinder (assuming same brake pedal travel and force) would push more fluid through the lines and increase pressure.

Please help me understand if I am incorrect,

thx
lugnuts

 

[Bystander]

Pressure is NOT a function of volume in this case; it IS a function of the cylinder x-section area, larger area for constant applied pedal force means LOWER pressure.

 

[mrspeedybob]

Pressure is force/area. The are of a circle is πr², so, the area of the 17mm piston is 227 mm². The area of the 19mm piston would be 284 mm². Since there are 1 million mm² per m², Newtons per mm² would be mega-pascals. If you + the brake booster apply 1000 Newtons to the piston the 17mm piston will produce 1000/227=4.41 mpa. The same 1000 Newtons applied to the 19mm piston would only produce 1000/284=3.52 mpa.

A brake system is not a completely static system however, pieces move. Therefore a certain volume of fluid must be moved in order to apply the brakes. The 19mm piston will move more fluid for the same amount of pedal travel, therefore you will not have to push the pedal as far to apply the brakes, you will however, have to push it harder.