The measure of the braking effect is also determined by the contact pressure of the pad. The contact pressure of the pad increases with the pressure applied to the brake pedal and the hydraulic pressure thus generated.
In the case of the brake disc, the moveable piston is sealed against the caliper housing (fluid part) by the so called sealing ring (see Fig. 1).
This pre-stressed sealing ring, which encompasses the brake caliper piston also serves to retract the piston when pressure decreases (“rollback”) and to re-adjust automatically the excessive clearance due to pad wear. Because of the rollback effect of the sealing ring, the piston will be retracted when pressure decreases.
In order to prevent the clearance from increasing during unbraked driving due to disc distortion, excessive disc runout or heavy vibration, a slight piston shift in the direction of the caliper housing is possible. The piston is always retracted to its initial position because of the “knockback” effect (see Fig.2).
With increasing clearance (due to wear) the piston has to travel a longer distance (than with new pads). The piston will slip through the sealing ring, as its retracting force is now less than the friction force.
Adjustment is therefore infinite and matched to the relevant rate of wear.
In order to protect the piston and cylinder surfaces from becoming dirty due to outside influences (road dust, pad wear debris, splash water, etc.), an additional dust seal (also known as a dust collar) is fitted. It must be ensured, that this is always in perfect condition and correctly fitted. Two disc brake pads are required per caliper, i.e., four pads per axle.