If you are replacing disc brake pads, then a disc brake spreader is a very useful tool. Spreaders push the caliper pistons back into the caliper with damaging the rotor or caliper. Depending on the design of the brakes on your car there are several different types of spreaders. This article covers how they work, and the different types available.
Once you remove the old disc brake pads from your caliper, you need to push the caliper piston, or pistons back into the caliper housing. Fundamentally hydraulic brakes are quite simple, as you press down on the brake pedal, you are actually pushing against what is called the master cylinder.
This motion then pushes brake fluid from the master cylinder out to each caliper where the caliper pistons (or slave cylinder) are pushed out the caliper body. This pistons being forced out of the caliper is what forces the brake pads to press into the rotor and slow the car.
When you take you foot off the brake pedal, the piston does not automatically retract back into the caliper like you might think. If it did this, then the next time you push the brakes, the piston would have to travel all way back out to contact the rotor again. This would make the brake pedal go to the floor, and for you to think that you have lost the brakes. This is exactly what happens right after you replace the brake pads and the pedal goes to the floor as you pump brakes. The system is resetting, and the pads are making their way back out to their normal resting spot just off the rotor. The normal position of the brake pads is a millimeter or less from the surface of the rotor.
So when you remove the old brake pads you need to push the caliper pistons back into the caliper, otherwise you cannot fit the thick new pads and the rotor into the caliper housing as you replace it over the rotor. This is typically a straight forward process if you have the correct tools. For both Subaru and Volvo's (which I am focusing on here), you simply need to apply enough pressure to the caliper piston to press it back into the caliper all the way.
Some manufacturers actually have a ratcheting system that physically prevents the piston from receding back into the caliper unless it is rotated, also known as "spin back". The ratcheting system is part of the parking brake system, so it is typically found on single piston, rear brakes. Two easy ways to identify if your car has this a spin back system is that the piston will look more like the bottom of a coffee cup (instead of the inside of the cup) as shown below and the parking brake cable will lead directly into the caliper, instead of the wheel spindle.
The key concern when pressing the piston back into the caliper is to do it slowly, and to make sure that you apply uniform pressure across the face of the piston (or to the center), as opposed to pressing in on only one side of the piston. If you press on one side of the piston, it will cause the piston to tilt as it retracts back into the caliper. This tilting will score and scratch the caliper housing, causing the whole system to leak in the future.
For these reasons a set of tools are available, known as disc brake pad spreaders, or disc brake compression tools. Depending on the design of your brakes, there is a tool available. Spreader designs that will accommodate a number of different makes is the single/dual piston design are shown below.
Single Piston Spreader
Single & Dual Piston Spreader
If you have high performance brakes, known as multi-pot caliper, such as equipped on the Subaru STi's, or Volvo R's then the standard compressors shown above will not work for you. In these cases you need to multi-pot spreader, or what Lisle calls a quad brake spreader. This type of tool is more expensive, but will work for all brake calipers, excepting the rotating type mentioned before.
Regardless of the style and type of spreader you use, it is recommended that you use your old pads, between the pistons and the tool itself. By using your old pads, you are spreading out the pressure across the face of the pistons. By spreading out the force, you are far less likely to tilt the pistons as they are pressed back into the caliper.
If you have a caliper that requires rotating to allow it to be retracted, then you'll need a tool set such as the one shown below. There are a number of lower cost alternatives, such as using needle nose pliers, or a cube looking tool from Lisle, but make sure that it will fit your brakes before buying them.