As I have a longer than standard Eurosport motor in a 78 frame
its something I've experimented with and as said
Its best to have it or something like it.
To be precise, its not a reservoir exactly, but an expansion bottle to hold the
coolant when it expands as the motor gets hot.
The rad cap has a two way valve that allows coolant to get drawn back again when the
motor cools down and the coolant contracts.
It is rather Fugly isnt it? I put mine down inside the crash bars
where no one would notice it