Hello Douglas,
Firstly the logic of the valve sizes. the blower and injector valves will get a lot of use so I have used 1/4″ ME size. The brake isolating valve needs to be smaller to just squeeze in the space and won’t see much use. There are different size nipples commercially available to suit the pipe size so you can match a 1/8″ pipe to a 1/4″ ME valve. I also sometimes do what LBSC recommended and solder a ring straight on the pipe so that a bigger pipe size can be used on a particular valve!
I’ll be covering this in the series shortly, thanks for raising the point.
Nick