G'day Norm,
The notched disc arrangement was used from the very beginning and thro' to the tractor models. I doubt that it's a coincidence that the brake design changed at the launch of the mecano models and that the first Australian Standard for ride-ons was published in 1990. This standard required (requires?) a service brake be fitted and specifies the minimum performance standard- which isn't great.
I've never mucked around with the later brake arrangement so really can't comment on its design or repairability. To be honest I've never felt the need for any sort of brake on my Greenies but it must be said that I've always worked on relatively flat ground.
Cheers,
