Originally Posted by dr bob

Something may have been lost in the original translation, but.. the two pistons in the master cylinder normally sit a bit apart. The primary piston (rear in the cylinder) actuates the front circuits, and is pushed directly by the pedal via the booster. The front piston in the cylinder is pushed by hydraulic pressure from the rear piston. If the rear brake circuit fails, the rear piston will ultimately push the front piston mechanically to its stop, and then build pressure to actuate the front brake circuits. Similarly, a failure in the front circuits causes the rear piston to push mechanically against the front piston, building pressure in the rear braking circuit. The front piston to rear brakes design can make the description a little confusing. Add in a check-valve that allows flow from the front piston bleed port to fill between the two pistons when they are fully retracted.

The system can be quite sensitive to bleed order, for instance, if you are doing it by the manual push-and-squirt method. It's also quite possible to get air in between the two pistons that's really tough to get out without removing and bench-bleeding the MC full-stroke, something not really recommended for older MC's where the might be corrosion or crud in the front of the bore.

Hope this makes a little bit of sense.

Originally Posted by dr bob

And this I'd different if you have flats vs spoke wheels. There's no magic to the red stud, except that it's used to make sure the factory-balanced wheel and tire goes back the same way every time. Makes vibration diagnosis more consistent. Once you fit new tires, that benefit is gone. It's still handy for you for the same reason, bit it could be any stud or any index with the valves and locks really. And you don't really need to use red...