Brake fluid leaking really bad

It’s been sitting for awhile because it needs a new transmission and when I started it today to move it out of the drive way it was smoking under the hood and fluid was leaking really bad. I have found out it’s brake fluid and it’s profusely leaking out of this curved rubber tube with a cap at the back of the engine bay. but I don’t know what that part is called and I cannot find it anywhere. I need help finding out what it is called and why it would be leaking so badly. It’s located right underneath the thermostat.