How do i push the rear caliper piston back?

To push the rear caliper piston back, you'll need a caliper tool or a C-clamp. First, remove the wheel and brake pads to access the caliper. Then, insert the tool or place the C-clamp over the caliper with one side against the piston and the other against the caliper body, and gently tighten it to push the piston back into the caliper. Make sure to check the brake fluid reservoir, as pushing the piston back may cause fluid to rise and overflow.