Find that hard to believe!... single circuit brakes are better than dual circuit??
I've driven a truck with only the front brakes when the rear ones failed, and I sure knew the difference, like plan stops a block ahead of time.
He is correct in saying that the original system in reasonable shape will give no problems, if it's in good shape it will work fine, by definition. It won't always be in perfect shape, and the brake system still has a chance of failure with the most careful maintenance.
It's the failure mode on single circuit brakes that sucks, you have nothing.
With dial circuit brakes if something fails likely you still have some braking ability; some or even the chance of some is better than none at all.
At least that's how I see it,
Grigg
