Seconded.
When I built my Spyder I had a brand new brake pipe that had been fitted by a previous owner come flying out of the union feeding the rear brakes while I was bleeding them. It hadn't been flared properly and blew out of its union when I stamped on the middle pedal to check if there was any air left in the system.
If that had happened on the road it would have had catastrophic and possibly fatal consequences as, being a single-circuit system, it would have caused total brake failure
My Speedster and all subsequent builds are getting dual-circuit brakes...