There was a tool that was like a lever with a pivotting link attached.
The lever end was 'anchored' against the backplate and the (wire) link hooked around the end of the spring then, using the leverage advantage of the tool the end of the spring was pulled away from the shoe.
The manoeuvre was reversed to fit the springs.
It would be easy to lash-up a tool like this - you just need a sturdy piece of metal for the lever and a suitable length of wire with an eye one end and a hook at the other.
This wire needs to be hardened after shaping.
The eye end of the wire attaches to the lever via a 'stud' or bolt (such that it is free to pivot).
The distance from the end of the lever depends on the size of the brake and the strength of the springs.
I'm sorry that I cannot find a photograph.
This is the nearest that I can find:-
http://www.fiskars.eu/var/fiskars_em...rning-Hook.jpg