Is it bad to push the caliper piston in and let brake fluid go back into master cylinder on new cars?

Yes, it can be bad to push the caliper piston in and let brake fluid go back into the master cylinder on new cars. This is because the brake fluid in new cars is typically very clean and free of contaminants.

When you push the caliper piston in, you can introduce air and dirt into the brake fluid, which can cause problems with the brake system. Additionally, the brake fluid in new cars is often under a lot of pressure, and pushing the caliper piston in can cause this pressure to build up and damage the brake system.