How do you depress brake piston without tool?

To depress a brake piston without a tool, you can use a large C-clamp or a similar object to gently push the piston back into the brake caliper. Place a piece of wood or a thick cloth between the clamp and the piston to protect it from damage. Slowly tighten the clamp, ensuring even pressure, until the piston is fully retracted. Always check the fluid level in the brake reservoir as you do this, as it may rise and overflow.