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RE: [T3] Master cylinder question

On 14 Nov 2001, at 6:20, Allyn de Vars wrote:

> The MC for disk brakes relieves all pressure to prevent the pads from
> rubbing against the brake disk while driving.  In a drum brake system, there
> are springs which cause the shoes to retract to their normal position.

Right, so while drum brakes can tolerate a bit of residual pressure, disks 
can't, but that doesn't mean that drums need the extra bit of pressure. It's 
clear that they do just fine without it.

> The larger diameter master cylinder will increase the amount of fluid pushed
> into the system when the brake pedal is applied, so you should find the
> brakes slightly more touchy than they used to be, but they will work. 

The first part of your statement is correct, but this will actually make the 
brakes LESS touchy. This is subtle, so bear with me a moment.

The larger piston has a larger frontal area, so the FORCE applied to the fluid 
is now spread over more area, giving lower hydraulic Pressure. Since the 
braking force at the wheels is dependent on the PRESSURE there, we now 
get LESS braking for the same pedal effort (FORCE.) The fact that the piston 
moves more fluid while exerting less pressure is just the normal 
consequence of the reduced mechanical advantage. This WILL make the 
pedal feel firm sooner. In fact it will make it feel firmer, but this just reflects 
the fact that while our foot now has less hydraulic advantage on the wheels, 
the wheels now have more advantage on our foot. In the end, we will now 
have to press harder on the pedal to get the same amount of braking action.

It's the same as what we get with a lever and fulcrum: More travel on the 
output end means less force available there.

Jim Adney
Madison, WI 53711-3054

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