There are a few ways that you can press a brake caliper piston without using a tool. One way is to use a C-clamp. You will need to place the C-clamp over the brake caliper piston and then tighten it until the piston is fully pressed in. Another way is to use a piece of wood or a block of metal that is slightly smaller than the piston. You will need to place this object over the piston and then hit it with a hammer until the piston is fully pressed in.