I'm not sure I've read this correctly. Are you asking; when the motorcycle is at rest (perhaps on the center-stand), and the rear brake pedal is depressed do all three pistons in the rear caliper react?
The answer is no; if the SMC (secondary master cylinder) is operating properly. However my experiences with a corroded SMC is that yes it is possible for the rear brake pedal to unintentionally cause all three pistons (of the rear brake caliper) to react. I've made a
sketch of the SMC that I rebuilt.
When the SMC piston is prevented from fully retracting, the primary seal blocks the compensating port, which in turn prevents brake fluid from passing back to the 'supply' side of the hydraulic circuit. With brake fluid 'trapped' between the SMC and the rear brake caliper, and no where to go as the brake fluid expands, the rear brake caliper pistons begin to creep out causing the rear brake pads to drag.
So what about the two outside rear brake caliper pistons?
Well, imagine a motorcycle that has sat all night with a rear wheel that can spin, and a corroded SMC. What happens when the brake pedal is applied for the first time?
- The rear brake caliper receives pressurized brake fluid causing the center piston to activate as usual, and indeed the center piston will retract as usual when brake pedal pressure is removed.
- The SMC also receives pressurized brake fluid, but due to the compensating port being blocked, the pressurized brake fluid that leaks past the primary seal causes the two outside pistons of the rear brake caliper to activate. Unfortunately (again due to the blocked compensating port), when brake pedal pressure is removed the two outside rear brake caliper pistons do not retract, but instead remain activated causing the rear brake pads to drag.
My experiences with the SMC on my motorcycle leads me to believe that while this is a very critical component of the hydraulic brake system, it is also very sensitive to the effects of it's environment, requiring a greater amount of care and maintenance then other hydraulic brake systems I've encountered.