Go to a truck parts supplier and get a LeverLock setup. It's DOT approved stuff, and works right. Just flip the lever and hit the brake, the valve holds pressure till you flip the lever back. And they have a bleeder in them. Great West or Brake and Drive in Swift should have them. There's an electric one, too, that doesn't require all the plumbing. Brake fluid might be hard on the seals in a ball valve.