Fixed Caliper Brake
The fixed caliper comprises two half calipers (flange and cover), each of which have one or two brake pistons). Both parts, firmly screwed together (expansion screws) and connected by the so-called channel bore, make up the “fixed caliper”.
This is bolted to the wheel suspension of the vehicle (kingpin, axle flange) or in the case of inboard brakes, to the gearbox.
When the brake is operated (build-up of hydraulic pressure), two or four brake pistons respectively force the pads to simultaneously make contact with the rotating brake discs on each side (braking position). The brake pads are guided and supported in the so-called caliper housing.
The caliper housing must be clean and undamaged, in order to prevent the brake pads from jamming or seizing up due to rust. If this happened, it would be impossible to press the pads against the brake discs (no braking effect).
When the pressure falls (upon completion of braking) the brake pistons are retracted due to the “rollback” – as described under “Function of the disc brake” – and the brake pads are forced against the piston by the expansion spring. The brake disc can now rotate freely because of the clearance.
To compensate for tapered brake pad wear on the leading edge, some fixed calipers have a “piston shoulder”. This piston shoulder must always be in its specified position with respect to the leading edge (observe garage manual instructions).
If this is not the case, it can not fulfil its function and it can lead to unpleasant brake noise.
If the piston is not aligned correctly, it must be returned to the specified position using piston turning pliers and the prescribed piston gauge.
When replacing the brake pads, the pad retaining pins and expansion springs must be removed after the wheels have been dismounted.
As the pistons are in an advanced position due to pad wear, they have to be pushed back using a piston retraction device (the pushed back brake fluid will cause the fluid level in the reservoir to rise).
After fitting the new brake pads, expansion springs and pad retaining pins, the brake pedal must be operated several times to optimize the caliper clearance, before taking the vehicle for a test drive