Brake line pressures

[kenmorton]

Looking on the AP web site gives brake line pressures for various sized "single" master cylinders at 50lb pedal pressure and 5:1 pedal ratio.
So am I right in assuming that these line pressures would be halved if twin cylinders of the same size were used with a balance bar set exactly 50/50 ?
And what would the primary/secondary pressures be in the same bores etc. tandem master cylinder?

[GreenV8S]

You would typically keep the line pressure the same but select the master cylinder size to give you the required pedal weight. Double the number of pistons means they each need to be half the area to get the same pedal weight for a given line pressure.

One you go to twin, you have the opportunity to run the front and rear circuits at different line pressures, and you'd want to choose the master cylinder sizes to get that ratio about right (and then use the balance bar to fine-tune the balance). The total hydro-mechanical leverage between the pedal and the road needs to be the same in all cases if you want the same pedal weight i.e. pedal leverage * ratio of slave/master piston areas * ratio of calliper effective radius to wheel rolling radius.