The two pistons in the master cylinder are not rigidly connected. Each piston has a return spring with the primary piston spring between the two pistons. Stepping on the brake pedal moves a pushrod, causing the first or primary piston to move forward. The fluid ahead of it cannot be compressed, so the secondary piston moves. As the pistons progress deeper into the cylinder bore, the brake fluid that is put under pressure transmits this force through both systems to friction pads at the wheels. A retaining ring fits into a groove near the end of the bore and holds the piston inside the cylinder.